The 2006 observations by the Spitzer Space Telescope revealed that the Andromeda Galaxy contains approximately one trillion stars,[10] more than twice the number of the Milky Way’s estimated 200-400 billion stars,[13] the Andromeda Galaxy, spanning approximately 220,000 light years, is the largest galaxy in our Local Group, which is also home to the Triangulum Galaxy and other minor galaxies. The Andromeda Galaxy's mass is estimated to be around 1.76 times that of the Milky Way Galaxy (~0.8-1.5×1012 solar masses [9][10] vs the Milky Way's 8.5×1011 solar masses).

Around the year 964, the Persian astronomer Abd al-Rahman al-Sufi described the Andromeda Galaxy, in his Book of Fixed Stars as a "nebulous smear".[18]Star charts of that period labeled it as the Little Cloud.[19] In 1612, the German astronomer Simon Marius gave an early description of the Andromeda Galaxy based on telescopic observations,[20] the German philosopher Immanuel Kant in 1755 in his work Universal Natural History and Theory of the Heavens conjectured that the blurry spot was an island universe. In 1764, Charles Messier cataloged Andromeda as object M31 and incorrectly credited Marius as the discoverer despite it being visible to the naked eye; in 1785, the astronomer William Herschel noted a faint reddish hue in the core region of Andromeda. He believed Andromeda to be the nearest of all the "great nebulae", and based on the color and magnitude of the nebula, he incorrectly guessed that it is no more than 2,000 times the distance of Sirius;[21] in 1850, William Parsons, 3rd Earl of Rosse, saw and made the first drawing of Andromeda's spiral structure.

In 1864, William Huggins noted that the spectrum of Andromeda differs from a gaseous nebula,[22] the spectra of Andromeda displays a continuum of frequencies, superimposed with dark absorption lines that help identify the chemical composition of an object. Andromeda's spectrum is very similar to the spectra of individual stars, and from this, it was deduced that Andromeda has a stellar nature; in 1885, a supernova (known as S Andromedae) was seen in Andromeda, the first and so far only one observed in that galaxy. At the time Andromeda was considered to be a nearby object, so the cause was thought to be a much less luminous and unrelated event called a nova, and was named accordingly; "Nova 1885".[23]

In 1887, Isaac Roberts took the first photographs of Andromeda, which was still commonly thought to be a nebula within our galaxy. Roberts mistook Andromeda and similar spiral nebulae as solar systems being formed.[citation needed] In 1912, Vesto Slipher used spectroscopy to measure the radial velocity of Andromeda with respect to our solar system—the largest velocity yet measured, at 300 kilometres per second (190 mi/s).[24]

Location of the Andromeda Galaxy (M31) in the Andromeda constellation.

In 1917, Heber Curtis observed a nova within Andromeda. Searching the photographic record, 11 more novae were discovered. Curtis noticed that these novae were, on average, 10 magnitudes fainter than those that occurred elsewhere in the sky, as a result, he was able to come up with a distance estimate of 500,000 light-years (3.2×1010 AU). He became a proponent of the so-called "island universes" hypothesis, which held that spiral nebulae were actually independent galaxies.[25]

In 1920, the Great Debate between Harlow Shapley and Curtis took place, concerning the nature of the Milky Way, spiral nebulae, and the dimensions of the universe. To support his claim of the Great Andromeda Nebula being, in fact, an external galaxy, Curtis also noted the appearance of dark lanes within Andromeda which resembled the dust clouds in our own galaxy, as well as historical observations of Andromeda Galaxy's significant Doppler shift; in 1922 Ernst Öpik presented a method to estimate the distance of Andromeda using the measured velocities of its stars. His result placed the Andromeda Nebula far outside our galaxy at a distance of about 450,000 parsecs (1,500,000 ly).[27]Edwin Hubble settled the debate in 1925 when he identified extragalactic Cepheid variable stars for the first time on astronomical photos of Andromeda. These were made using the 2.5-metre (100-in) Hooker telescope, and they enabled the distance of Great Andromeda Nebula to be determined. His measurement demonstrated conclusively that this feature is not a cluster of stars and gas within our own Galaxy, but an entirely separate galaxy located a significant distance from the Milky Way.[28]

In 1943, Walter Baade was the first person to resolve stars in the central region of the Andromeda Galaxy. Baade identified two distinct populations of stars based on their metallicity, naming the young, high-velocity stars in the disk Type I and the older, red stars in the bulge Type II, this nomenclature was subsequently adopted for stars within the Milky Way, and elsewhere. (The existence of two distinct populations had been noted earlier by Jan Oort.)[29] Baade also discovered that there were two types of Cepheid variables, which resulted in a doubling of the distance estimate to Andromeda, as well as the remainder of the Universe.[30]

The estimated distance of the Andromeda Galaxy from our own was doubled in 1953 when it was discovered that there is another, dimmer type of Cepheid; in the 1990s, measurements of both standard red giants as well as red clump stars from the Hipparcos satellite measurements were used to calibrate the Cepheid distances.[35][36]

Andromeda Galaxy was formed roughly 10 billion years ago from the collision and subsequent merger of smaller protogalaxies,[37] this violent collision formed most of the galaxy's (metal-rich) galactic halo and extended disk. During this epoch, star formation would have been very high, to the point of becoming a luminous infrared galaxy for roughly 100 million years. Andromeda and the Triangulum Galaxy had a very close passage 2–4 billion years ago, this event produced high levels of star formation across the Andromeda Galaxy's disk – even some globular clusters – and disturbed M33's outer disk.

Over the past 2 billion years, star formation throughout Andromeda's disk is thought to have decreased to the point of near-inactivity. There have been interactions with satellite galaxies like M32, M110, or others that have already been absorbed by Andromeda Galaxy, these interactions have formed structures like Andromeda's Giant Stellar Stream. A galactic merger roughly 100 million years ago is believed to be responsible for a counter-rotating disk of gas found in the center of Andromeda as well as the presence there of a relatively young (100 million years old) stellar population.[citation needed]

At least four distinct techniques have been used to estimate distances from Earth to the Andromeda Galaxy.

In 2003, using the infrared surface brightness fluctuations (I-SBF) and adjusting for the new period-luminosity value and a metallicity correction of −0.2 mag dex−1 in (O/H), an estimate of 2.57 ± 0.06 million light-years (1.625×1011 ± 3.8×109AU) was derived.

In 2004, using the Cepheid variable method, the distance was estimated to be 2.51 ± 0.13 million light-years (770 ± 40 kpc).[2][3]

In 2005, an eclipsing binary star was discovered in the Andromeda Galaxy, the binary[c] is two hot blue stars of types O and B. By studying the eclipses of the stars, astronomers were able to measure their sizes. Knowing the sizes and temperatures of the stars, they were able to measure their absolute magnitude. When the visual and absolute magnitudes are known, the distance to the star can be measured, the stars lie at a distance of 2.52×10^6 ± 0.14×10^6 ly (1.594×1011 ± 8.9×109 AU) and the whole Andromeda Galaxy at about 2.5×10^6 ly (1.6×1011 AU).[4] This new value is in excellent agreement with the previous, independent Cepheid-based distance value.

In 2005, using the TRGB method, the distance was estimated to be 2.56×10^6 ± 0.08×10^6 ly (1.619×1011 ± 5.1×109 AU).[5]

Averaged together, these distance estimates give a value of 2.54×10^6 ± 0.11×10^6 ly (1.606×1011 ± 7.0×109 AU).[a] And, from this, the diameter of Andromeda at the widest point is estimated to be 220 ± 3 kly (67,450 ± 920 pc).[original research?] Applying trigonometry (angular diameter), this is equivalent to an apparent 4.96° angle in the sky.

Mass estimates for the Andromeda Galaxy's halo (including dark matter) give a value of approximately 1.5×1012M☉[10] (or 1.5 trillionsolar masses) compared to 8×1011M☉ for the Milky Way. This contradicts earlier measurements, that seem to indicate that Andromeda Galaxy and the Milky Way are almost equal in mass. Even so, Andromeda Galaxy's spheroid actually has a higher stellar density than that of the Milky Way[39] and its galactic stellar disk is about twice the size of that of the Milky Way,[11] the total stellar mass of Andromeda Galaxy is estimated to be between 1.1×1011M☉.,[40][41] (i.e., around twice as massive as that of the Milky Way) and 1.5×1011M☉, with around 30% of that mass in the central bulge, 56% in the disk, and the remaining 14% in the halo.[42]

Andromeda Galaxy is surrounded by a large and massive halo of hot gas that is estimated to contain half the mass of the stars in the galaxy, the nearly invisible halo stretches about a million light-years from its host galaxy, halfway to our Milky Way galaxy. Simulations of galaxies indicate the halo formed at the same time as the Andromeda Galaxy, the halo is enriched in elements heavier than hydrogen and helium, formed from supernovae and its properties are those expected for a galaxy that lies in the "green valley" of the Galaxy color–magnitude diagram (see below). Supernovae erupt in Andromeda Galaxy's star-filled disk and eject these heavier elements into space, over Andromeda Galaxy's lifetime, nearly half of the heavy elements made by its stars have been ejected far beyond the galaxy's 200,000-light-year-diameter stellar disk.[45][46][47][48][49]

Compared to the Milky Way, the Andromeda Galaxy appears to have predominantly older stars with ages >7×109 years.[42][clarification needed] The estimated luminosity of Andromeda Galaxy, ~2.6×1010L☉, is about 25% higher than that of our own galaxy.[50] However, the galaxy has a high inclination as seen from Earth and its interstellar dust absorbs an unknown amount of light, so it is difficult to estimate its actual brightness and other authors have given other values for the luminosity of the Andromeda Galaxy (some authors even propose it is the second-brightest galaxy within a radius of 10 mega-parsecs of the Milky Way, after the Sombrero Galaxy,[51] with an absolute magnitude of around -22.21[d] or close[52]).

The rate of star formation in the Milky Way is much higher, with Andromeda Galaxy producing only about one solar mass per year compared to 3–5 solar masses for the Milky Way, the rate of supernovae in the Milky Way is also double that of Andromeda Galaxy.[54][not in citation given] This suggests that the latter once experienced a great star formation phase, but is now in a relative state of quiescence, whereas the Milky Way is experiencing more active star formation.[50] Should this continue, the luminosity of the Milky Way may eventually overtake that of Andromeda Galaxy.

According to recent studies, the Andromeda Galaxy lies in what in the galaxy color–magnitude diagram is known as the "green valley", a region populated by galaxies like the Milky Way in transition from the "blue cloud" (galaxies actively forming new stars) to the "red sequence" (galaxies that lack star formation). Star formation activity in green valley galaxies is slowing as they run out of star-forming gas in the interstellar medium; in simulated galaxies with similar properties to Andromeda Galaxy, star formation is expected to extinguish within about five billion years from the now, even accounting for the expected, short-term increase in the rate of star formation due to the collision between Andromeda Galaxy and the Milky Way.[55]

A Galaxy Evolution Explorer image of the Andromeda Galaxy. The bands of blue-white making up the galaxy's striking rings are neighborhoods that harbor hot, young, massive stars. Dark blue-grey lanes of cooler dust show up starkly against these bright rings, tracing the regions where star formation is currently taking place in dense cloudy cocoons. When observed in visible light, Andromeda Galaxy’s rings look more like spiral arms, the ultraviolet view shows that these arms more closely resemble the ring-like structure previously observed in infrared wavelengths with NASA’s Spitzer Space Telescope. Astronomers using the latter interpreted these rings as evidence that the galaxy was involved in a direct collision with its neighbor, M32, more than 200 million years ago.

In 2005, astronomers used the Keck telescopes to show that the tenuous sprinkle of stars extending outward from the galaxy is actually part of the main disk itself,[11] this means that the spiral disk of stars in the Andromeda Galaxy is three times larger in diameter than previously estimated. This constitutes evidence that there is a vast, extended stellar disk that makes the galaxy more than 220,000 light-years (67,000 pc) in diameter. Previously, estimates of the Andromeda Galaxy's size ranged from 70,000 to 120,000 light-years (21,000 to 37,000 pc) across.

The galaxy is inclined an estimated 77° relative to the Earth (where an angle of 90° would be viewed directly from the side). Analysis of the cross-sectional shape of the galaxy appears to demonstrate a pronounced, S-shaped warp, rather than just a flat disk.[57] A possible cause of such a warp could be gravitational interaction with the satellite galaxies near the Andromeda Galaxy, the Galaxy M33 could be responsible for some warp in Andromeda's arms, though more precise distances and radial velocities are required.

Spectroscopic studies have provided detailed measurements of the rotational velocity of the Andromeda Galaxy as a function of radial distance from the core. The rotational velocity has a maximum value of 225 kilometres per second (140 mi/s) at 1,300 light-years (82,000,000 AU) from the core, and it has its minimum possibly as low as 50 kilometres per second (31 mi/s) at 7,000 light-years (440,000,000 AU) from the core. Further out, rotational velocity rises out to a radius of 33,000 light-years (2.1×109AU), where it reaches a peak of 250 kilometres per second (160 mi/s). The velocities slowly decline beyond that distance, dropping to around 200 kilometres per second (120 mi/s) at 80,000 light-years (5.1×109AU). These velocity measurements imply a concentrated mass of about 6×109M☉ in the nucleus. The total mass of the galaxy increases linearly out to 45,000 light-years (2.8×109AU), then more slowly beyond that radius.[58]

The spiral arms of the Andromeda Galaxy are outlined by a series of H II regions, first studied in great detail by Walter Baade and described by him as resembling "beads on a string". His studies show two spiral arms that appear to be tightly wound, although they are more widely spaced than in our galaxy,[59] his descriptions of the spiral structure, as each arm crosses the major axis of the Andromeda Galaxy, are as follows[60]§pp1062[61]§pp92:

Baade's spiral arms of M31

Arms (N=cross M31's major axis at north, S=cross M31's major axis at south)

Since the Andromeda Galaxy is seen close to edge-on, it is difficult to study its spiral structure. Rectified images of the galaxy seem to show a fairly normal spiral galaxy, exhibiting two continuous trailing arms that are separated from each other by a minimum of about 13,000 light-years (820,000,000 AU) and that can be followed outward from a distance of roughly 1,600 light-years (100,000,000 AU) from the core. Alternative spiral structures have been proposed such as a single spiral arm[62] or a flocculent[63] pattern of long, filamentary, and thick spiral arms.[1][64]

The most likely cause of the distortions of the spiral pattern is thought to be interaction with galaxy satellites M32 and M110,[65] this can be seen by the displacement of the neutral hydrogen clouds from the stars.[66]

In 1998, images from the European Space Agency's Infrared Space Observatory demonstrated that the overall form of the Andromeda Galaxy may be transitioning into a ring galaxy. The gas and dust within the galaxy is generally formed into several overlapping rings, with a particularly prominent ring formed at a radius of 32,000 light-years (2.0×109AU) (10 kiloparsecs) from the core,[67] nicknamed by some astronomers the ring of fire.[68] This ring is hidden from visible light images of the galaxy because it is composed primarily of cold dust, and most of the star formation that is taking place in the Andromeda Galaxy is concentrated there.[69]

Later studies with the help of the Spitzer Space Telescope showed how Andromeda Galaxy's spiral structure in the infrared appears to be composed of two spiral arms that emerge from a central bar and continue beyond the large ring mentioned above, those arms, however, are not continuous and have a segmented structure.[65]

Close examination of the inner region of the Andromeda Galaxy with the same telescope also showed a smaller dust ring that is believed to have been caused by the interaction with M32 more than 200 million years ago. Simulations show that the smaller galaxy passed through the disk of the Andromeda Galaxy along the latter's polar axis, this collision stripped more than half the mass from the smaller M32 and created the ring structures in Andromeda.[70] It is the co-existence of the long-known large ring-like feature in the gas of Messier 31, together with this newly discovered inner ring-like structure, offset from the barycenter, that suggested a nearly head-on collision with the satellite M32, a milder version of the Cartwheel encounter.[71]

Studies of the extended halo of the Andromeda Galaxy show that it is roughly comparable to that of the Milky Way, with stars in the halo being generally "metal-poor", and increasingly so with greater distance,[39] this evidence indicates that the two galaxies have followed similar evolutionary paths. They are likely to have accreted and assimilated about 100–200 low-mass galaxies during the past 12 billion years.[72] The stars in the extended halos of the Andromeda Galaxy and the Milky Way may extend nearly one-third the distance separating the two galaxies.

M31 is known to harbor a dense and compact star cluster at its very center; in a large telescope it creates a visual impression of a star embedded in the more diffuse surrounding bulge. In 1991, the Hubble Space Telescope was used to image Andromeda Galaxy's inner nucleus, the nucleus consists of two concentrations separated by 1.5 parsecs (4.9 ly). The brighter concentration, designated as P1, is offset from the center of the galaxy, the dimmer concentration, P2, falls at the true center of the galaxy and contains a black hole measured at 3–5 × 107M☉ in 1993,[73] and at 1.1–2.3 × 108M☉ in 2005.[74] The velocity dispersion of material around it is measured to be ≈ 160 km/s.[75]

Chandra X-ray telescope image of the center of Andromeda Galaxy. A number of X-ray sources, likely X-ray binary stars, within the galaxy's central region appear as yellowish dots, the blue source at the center is at the position of the supermassive black hole.

It has been proposed that the observed double nucleus could be explained if P1 is the projection of a disk of stars in an eccentric orbit around the central black hole,[76] the eccentricity is such that stars linger at the orbital apocenter, creating a concentration of stars. P2 also contains a compact disk of hot, spectral class A stars, the A stars are not evident in redder filters, but in blue and ultraviolet light they dominate the nucleus, causing P2 to appear more prominent than P1.[77]

While at the initial time of its discovery it was hypothesized that the brighter portion of the double nucleus is the remnant of a small galaxy "cannibalized" by Andromeda Galaxy,[78] this is no longer considered a viable explanation, largely because such a nucleus would have an exceedingly short lifetime due to tidal disruption by the central black hole. While this could be partially resolved if P1 had its own black hole to stabilize it, the distribution of stars in P1 does not suggest that there is a black hole at its center.[76]

Apparently, by late 1968, no X-rays had been detected from the Andromeda Galaxy.[79] A balloon flight on October 20, 1970, set an upper limit for detectable hard X-rays from the Andromeda Galaxy.[80]

Multiple X-ray sources have since been detected in the Andromeda Galaxy, using observations from the European Space Agency's (ESA) XMM-Newton orbiting observatory. Robin Barnardet al. hypothesized that these are candidate black holes or neutron stars, which are heating the incoming gas to millions of kelvins and emitting X-rays. The spectrum of the neutron stars is the same as the hypothesized black holes but can be distinguished by their masses.[81]

There are approximately 460 globular clusters associated with the Andromeda Galaxy,[82] the most massive of these clusters, identified as Mayall II, nicknamed Globular One, has a greater luminosity than any other known globular cluster in the Local Group of galaxies.[83] It contains several million stars, and is about twice as luminous as Omega Centauri, the brightest known globular cluster in the Milky Way. Globular One (or G1) has several stellar populations and a structure too massive for an ordinary globular, as a result, some consider G1 to be the remnant core of a dwarf galaxy that was consumed by Andromeda in the distant past.[84] The globular with the greatest apparent brightness is G76 which is located in the south-west arm's eastern half.[19] Another massive globular cluster, named 037-B327 and discovered in 2006 as is heavily reddened by the Andromeda Galaxy's interstellar dust, was thought to be more massive than G1 and the largest cluster of the Local Group;[85] however, other studies have shown it is actually similar in properties to G1.[86]

Unlike the globular clusters of the Milky Way, which show a relatively low age dispersion, Andromeda Galaxy's globular clusters have a much larger range of ages: from systems as old as the galaxy itself to much younger systems, with ages between a few hundred million years to five billion years[88]

In 2005, astronomers discovered a completely new type of star cluster in the Andromeda Galaxy, the new-found clusters contain hundreds of thousands of stars, a similar number of stars that can be found in globular clusters. What distinguishes them from the globular clusters is that they are much larger—several hundred light-years across—and hundreds of times less dense, the distances between the stars are, therefore, much greater within the newly discovered extended clusters.[89]

Messier 32 is to the left of the center, Messier 110 is to the bottom-right of the center.

Like the Milky Way, the Andromeda Galaxy has satellite galaxies, consisting of 14 known dwarf galaxies, the best known and most readily observed satellite galaxies are M32 and M110. Based on current evidence, it appears that M32 underwent a close encounter with the Andromeda Galaxy in the past. M32 may once have been a larger galaxy that had its stellar disk removed by M31, and underwent a sharp increase of star formation in the core region, which lasted until the relatively recent past.[91]

M110 also appears to be interacting with the Andromeda Galaxy, and astronomers have found in the halo of the latter a stream of metal-rich stars that appear to have been stripped from these satellite galaxies.[92] M110 does contain a dusty lane, which may indicate recent or ongoing star formation.[93]

In 2006, it was discovered that nine of the satellite galaxies lie in a plane that intersects the core of the Andromeda Galaxy; they are not randomly arranged as would be expected from independent interactions. This may indicate a common tidal origin for the satellites.[94]

The Andromeda Galaxy is approaching the Milky Way at about 110 kilometres per second (68 mi/s).[95] It has been measured approaching relative to our Sun at around 300 kilometres per second (190 mi/s)[1] as the Sun orbits around the center of our galaxy at a speed of approximately 225 kilometres per second (140 mi/s). This makes the Andromeda Galaxy one of about 100 observable blueshifted galaxies.[96] Andromeda Galaxy's tangential or sideways velocity with respect to the Milky Way is relatively much smaller than the approaching velocity and therefore it is expected to collide directly with the Milky Way in about 4 billion years. A likely outcome of the collision is that the galaxies will merge to form a giant elliptical galaxy[97] or perhaps even a large disc galaxy.[15] Such events are frequent among the galaxies in galaxy groups, the fate of the Earth and the Solar System in the event of a collision is currently unknown. Before the galaxies merge, there is a small chance that the Solar System could be ejected from the Milky Way or join the Andromeda Galaxy.[98]

The Andromeda Galaxy is bright enough to be seen with the naked eye, even with some light pollution.[99] Andromeda is best seen during autumn nights in the Northern Hemisphere, when from mid-latitudes the galaxy reaches zenith (its highest point at midnight) so can be seen almost all night. From the Southern Hemisphere, it is most visible at the same months, that is in spring, and away from our equator does not reach a high altitude over the northern horizon, making it difficult to observe. Binoculars can reveal some larger structures and its two brightest satellite galaxies, M32 and M110.[100] An amateurtelescope can reveal Andromeda's disk, some of its brightest globular clusters, dark dust lanes and the large star cloud NGC 206.[101][102]

1.
Constellation
–
A constellation is formally defined as a region of the celestial sphere, with boundaries laid down by the International Astronomical Union. The constellation areas mostly had their origins in Western-traditional patterns of stars from which the constellations take their names, in 1922, the International Astronomical Union officially recognized the 88 modern constellations, which cover the entire sky. They began as the 48 classical Greek constellations laid down by Ptolemy in the Almagest, Constellations in the far southern sky are late 16th- and mid 18th-century constructions. 12 of the 88 constellations compose the zodiac signs, though the positions of the constellations only loosely match the dates assigned to them in astrology. The term constellation can refer to the stars within the boundaries of that constellation. Notable groupings of stars that do not form a constellation are called asterisms, when astronomers say something is “in” a given constellation they mean it is within those official boundaries. Any given point in a coordinate system can unambiguously be assigned to a single constellation. Many astronomical naming systems give the constellation in which an object is found along with a designation in order to convey a rough idea in which part of the sky it is located. For example, the Flamsteed designation for bright stars consists of a number, the word constellation seems to come from the Late Latin term cōnstellātiō, which can be translated as set of stars, and came into use in English during the 14th century. It also denotes 88 named groups of stars in the shape of stellar-patterns, the Ancient Greek word for constellation was ἄστρον. Colloquial usage does not draw a distinction between constellation in the sense of an asterism and constellation in the sense of an area surrounding an asterism. The modern system of constellations used in astronomy employs the latter concept, the term circumpolar constellation is used for any constellation that, from a particular latitude on Earth, never sets below the horizon. From the North Pole or South Pole, all constellations south or north of the equator are circumpolar constellations. In the equatorial or temperate latitudes, the term equatorial constellation has sometimes been used for constellations that lie to the opposite the circumpolar constellations. They generally include all constellations that intersect the celestial equator or part of the zodiac, usually the only thing the stars in a constellation have in common is that they appear near each other in the sky when viewed from the Earth. In galactic space, the stars of a constellation usually lie at a variety of distances, since stars also travel on their own orbits through the Milky Way, the star patterns of the constellations change slowly over time. After tens to hundreds of thousands of years, their familiar outlines will become unrecognisable, the terms chosen for the constellation themselves, together with the appearance of a constellation, may reveal where and when its constellation makers lived. The earliest direct evidence for the constellations comes from inscribed stones and it seems that the bulk of the Mesopotamian constellations were created within a relatively short interval from around 1300 to 1000 BC

2.
Andromeda (constellation)
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Andromeda is one of the 48 constellations listed by the 2nd-century Greco-Roman astronomer Ptolemy and remains one of the 88 modern constellations. Located north of the equator, it is named for Andromeda, daughter of Cassiopeia, in the Greek myth. Andromeda is most prominent during autumn evenings in the Northern Hemisphere, because of its northern declination, Andromeda is visible only north of 40° south latitude, for observers farther south it lies below the horizon. It is one of the largest constellations, with an area of 722 square degrees. This is over 1,400 times the size of the moon, 55% of the size of the largest constellation, Hydra. Its brightest star, Alpha Andromedae, is a star that has also been counted as a part of Pegasus, while Gamma Andromedae is a colorful binary. Only marginally dimmer than Alpha, Beta Andromedae is a red giant, the constellations most obvious deep-sky object is the naked-eye Andromeda Galaxy, the closest spiral galaxy to the Milky Way and one of the brightest Messier objects. Several fainter galaxies, including M31s companions M110 and M32, as well as the more distant NGC891, the Blue Snowball Nebula, a planetary nebula, is visible in a telescope as a blue circular object. Andromeda is the location of the radiant for the Andromedids, a meteor shower that occurs in November. The uranography of Andromeda has its roots most firmly in the Greek tradition, the stars that make up Pisces and the middle portion of modern Andromeda formed a constellation representing a fertility goddess, sometimes named as Anunitum or the Lady of the Heavens. Andromeda is known as the Chained Lady or the Chained Woman in English and it was known as Mulier Catenata in Latin and al-Marat al Musalsalah in Arabic. Offended at her remark, the nymphs petitioned Poseidon to punish Cassiopeia for her insolence, Andromedas panicked father, Cepheus, was told by the Oracle of Ammon that the only way to save his kingdom was to sacrifice his daughter to Cetus. Perseus and Andromeda then married, the myth recounts that the couple had nine children together – seven sons, after Andromedas death Athena placed her in the sky as a constellation, to honor her. Several of the neighboring constellations also represent characters in the Perseus myth and it is connected with the constellation Pegasus. Andromeda was one of the original 48 constellations formulated by Ptolemy in his 2nd-century Almagest, in which it was defined as a specific pattern of stars. She is typically depicted with α Andromedae as her head, ο and λ Andromedae as her chains, and δ, π, μ, Β, however, there is no universal depiction of Andromeda and the stars used to represent her body, head, and chains. Arab astronomers were aware of Ptolemys constellations, but they included a second constellation representing a fish at Andromedas feet, several stars from Andromeda and most of the stars in Lacerta were combined in 1787 by German astronomer Johann Bode to form Frederici Honores. It was designed to honor King Frederick II of Prussia, in 1922, the IAU defined its recommended three-letter abbreviation, And

3.
Right ascension
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Right ascension is the angular distance measured eastward along the celestial equator from the vernal equinox to the hour circle of the point in question. When combined with declination, these astronomical coordinates specify the direction of a point on the sphere in the equatorial coordinate system. Right ascension is the equivalent of terrestrial longitude. Both right ascension and longitude measure an angle from a direction on an equator. Right ascension is measured continuously in a circle from that equinox towards the east. Any units of measure could have been chosen for right ascension, but it is customarily measured in hours, minutes. Astronomers have chosen this unit to measure right ascension because they measure a stars location by timing its passage through the highest point in the sky as the Earth rotates. The highest point in the sky, called meridian, is the projection of a line onto the celestial sphere. A full circle, measured in units, contains 24 × 60 × 60 = 86 400s, or 24 × 60 = 1 440m. Because right ascensions are measured in hours, they can be used to time the positions of objects in the sky. For example, if a star with RA = 01h 30m 00s is on the meridian, sidereal hour angle, used in celestial navigation, is similar to right ascension, but increases westward rather than eastward. Usually measured in degrees, it is the complement of right ascension with respect to 24h and it is important not to confuse sidereal hour angle with the astronomical concept of hour angle, which measures angular distance of an object westward from the local meridian. The Earths axis rotates slowly westward about the poles of the ecliptic and this effect, known as precession, causes the coordinates of stationary celestial objects to change continuously, if rather slowly. Therefore, equatorial coordinates are inherently relative to the year of their observation, coordinates from different epochs must be mathematically rotated to match each other, or to match a standard epoch. The right ascension of Polaris is increasing quickly, the North Ecliptic Pole in Draco and the South Ecliptic Pole in Dorado are always at right ascension 18h and 6h respectively. The currently used standard epoch is J2000.0, which is January 1,2000 at 12,00 TT, the prefix J indicates that it is a Julian epoch. Prior to J2000.0, astronomers used the successive Besselian Epochs B1875.0, B1900.0, the concept of right ascension has been known at least as far back as Hipparchus who measured stars in equatorial coordinates in the 2nd century BC. But Hipparchus and his successors made their star catalogs in ecliptic coordinates, the easiest way to do that is to use an equatorial mount, which allows the telescope to be aligned with one of its two pivots parallel to the Earths axis

4.
Declination
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In astronomy, declination is one of the two angles that locate a point on the celestial sphere in the equatorial coordinate system, the other being hour angle. Declinations angle is measured north or south of the celestial equator, the root of the word declination means a bending away or a bending down. It comes from the root as the words incline and recline. Declination in astronomy is comparable to geographic latitude, projected onto the celestial sphere, points north of the celestial equator have positive declinations, while those south have negative declinations. Any units of measure can be used for declination, but it is customarily measured in the degrees, minutes. Declinations with magnitudes greater than 90° do not occur, because the poles are the northernmost and southernmost points of the celestial sphere, the Earths axis rotates slowly westward about the poles of the ecliptic, completing one circuit in about 26,000 years. This effect, known as precession, causes the coordinates of stationary celestial objects to change continuously, therefore, equatorial coordinates are inherently relative to the year of their observation, and astronomers specify them with reference to a particular year, known as an epoch. Coordinates from different epochs must be rotated to match each other. The currently used standard epoch is J2000.0, which is January 1,2000 at 12,00 TT, the prefix J indicates that it is a Julian epoch. Prior to J2000.0, astronomers used the successive Besselian Epochs B1875.0, B1900.0, the declinations of Solar System objects change very rapidly compared to those of stars, due to orbital motion and close proximity. This similarly occurs in the Southern Hemisphere for objects with less than −90° − φ. An extreme example is the star which has a declination near to +90°. Circumpolar stars never dip below the horizon, conversely, there are other stars that never rise above the horizon, as seen from any given point on the Earths surface. Generally, if a star whose declination is δ is circumpolar for some observer, then a star whose declination is −δ never rises above the horizon, as seen by the same observer. Likewise, if a star is circumpolar for an observer at latitude φ, neglecting atmospheric refraction, declination is always 0° at east and west points of the horizon. At the north point, it is 90° − |φ|, and at the south point, from the poles, declination is uniform around the entire horizon, approximately 0°. Non-circumpolar stars are visible only during certain days or seasons of the year, the Suns declination varies with the seasons. As seen from arctic or antarctic latitudes, the Sun is circumpolar near the summer solstice, leading to the phenomenon of it being above the horizon at midnight

5.
Redshift
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In physics, redshift happens when light or other electromagnetic radiation from an object is increased in wavelength, or shifted to the red end of the spectrum. Some redshifts are an example of the Doppler effect, familiar in the change of apparent pitches of sirens, a redshift occurs whenever a light source moves away from an observer. Finally, gravitational redshift is an effect observed in electromagnetic radiation moving out of gravitational fields. However, redshift is a common term and sometimes blueshift is referred to as negative redshift. Knowledge of redshifts and blueshifts has been applied to develop several terrestrial technologies such as Doppler radar and radar guns, Redshifts are also seen in the spectroscopic observations of astronomical objects. Its value is represented by the letter z, a special relativistic redshift formula can be used to calculate the redshift of a nearby object when spacetime is flat. However, in contexts, such as black holes and Big Bang cosmology. Special relativistic, gravitational, and cosmological redshifts can be understood under the umbrella of frame transformation laws, the history of the subject began with the development in the 19th century of wave mechanics and the exploration of phenomena associated with the Doppler effect. The effect is named after Christian Doppler, who offered the first known physical explanation for the phenomenon in 1842, the hypothesis was tested and confirmed for sound waves by the Dutch scientist Christophorus Buys Ballot in 1845. Doppler correctly predicted that the phenomenon should apply to all waves, before this was verified, however, it was found that stellar colors were primarily due to a stars temperature, not motion. Only later was Doppler vindicated by verified redshift observations, the first Doppler redshift was described by French physicist Hippolyte Fizeau in 1848, who pointed to the shift in spectral lines seen in stars as being due to the Doppler effect. The effect is called the Doppler–Fizeau effect. In 1868, British astronomer William Huggins was the first to determine the velocity of a moving away from the Earth by this method. In 1871, optical redshift was confirmed when the phenomenon was observed in Fraunhofer lines using solar rotation, about 0.1 Å in the red. In 1887, Vogel and Scheiner discovered the annual Doppler effect, in 1901, Aristarkh Belopolsky verified optical redshift in the laboratory using a system of rotating mirrors. The word does not appear unhyphenated until about 1934 by Willem de Sitter, perhaps indicating that up to point its German equivalent. Beginning with observations in 1912, Vesto Slipher discovered that most spiral galaxies, Slipher first reports on his measurement in the inaugural volume of the Lowell Observatory Bulletin. Three years later, he wrote a review in the journal Popular Astronomy, Slipher reported the velocities for 15 spiral nebulae spread across the entire celestial sphere, all but three having observable positive velocities

6.
Blueshift
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A blueshift is any decrease in wavelength, with a corresponding increase in frequency, of an electromagnetic wave, the opposite effect is referred to as redshift. In visible light, this shifts the color from the red end of the spectrum to the blue end, Doppler blueshift is caused by movement of a source towards the observer. The term applies to any decrease in wavelength and increase in frequency caused by relative motion, blazars are known to propel relativistic jets toward us, emitting synchrotron radiation and bremsstrahlung that appears blueshifted. Nearby stars such as Barnards Star are moving toward us, resulting in a very small blueshift, Doppler blueshift of distant objects with a high z can be subtracted from the much larger cosmological redshift to determine relative motion in the expanding universe. There are faraway active galaxies that show a blueshift in their emission lines, one of the largest blueshifts is found in the narrow-line quasar, PG 1543+489, which has a relative velocity of -1150 km/s. These types of galaxies are called blue outliers, gravitational potential Redshift Relativistic Doppler effect

7.
Radial velocity
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The radial velocity of an object with respect to a given point is the rate of change of the distance between the object and the point. That is, the velocity is the component of the objects velocity that points in the direction of the radius connecting the object. In astronomy, the point is taken to be the observer on Earth. In astronomy, radial velocity is measured to the first order of approximation by Doppler spectroscopy. The quantity obtained by this method may be called the barycentric radial-velocity measure or spectroscopic radial velocity, by contrast, astrometric radial velocity is determined by astrometric observations. A positive radial velocity indicates the distance between the objects is or was increasing, a radial velocity indicates the distance between the source and observer is or was decreasing. In many binary stars, the orbital motion usually causes radial velocity variations of several kilometers per second, as the spectra of these stars vary due to the Doppler effect, they are called spectroscopic binaries. Radial velocity can be used to estimate the ratio of the masses of the stars and it has been suggested that planets with high eccentricities calculated by this method may in fact be two-planet systems of circular or near-circular resonant orbit. When the star moves towards us, its spectrum is blueshifted, by regularly looking at the spectrum of a star—and so, measuring its velocity—it can be determined, if it moves periodically due to the influence of a companion. From the instrumental perspective, velocities are measured relative to the telescopes motion, in the case of spectroscopic measurements corrections of the order of ±20 cm/s with respect to aberration. Proper motion Peculiar velocity Relative velocity The Radial Velocity Equation in the Search for Exoplanets

8.
Cosmic distance ladder
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The cosmic distance ladder is the succession of methods by which astronomers determine the distances to celestial objects. A real direct distance measurement of an object is possible only for those objects that are close enough to Earth. The techniques for determining distances to more distant objects are all based on various measured correlations between methods that work at distances and methods that work at larger distances. Several methods rely on a candle, which is an astronomical object that has a known luminosity. The ladder analogy arises because no single technique can measure distances at all ranges encountered in astronomy, instead, one method can be used to measure nearby distances, a second can be used to measure nearby to intermediate distances, and so on. Each rung of the ladder provides information that can be used to determine the distances at the next higher rung, at the base of the ladder are fundamental distance measurements, in which distances are determined directly, with no physical assumptions about the nature of the object in question. The precise measurement of stellar positions is part of the discipline of astrometry, direct distance measurements are based upon the astronomical unit, which is the distance between the Earth and the Sun. Historically, observations of transits of Venus were crucial in determining the AU, in the first half of the 20th century, observations of asteroids were also important. Keplers laws provide precise ratios of the sizes of the orbits of objects orbiting the Sun, radar is used to measure the distance between the orbits of the Earth and of a second body. From that measurement and the ratio of the two sizes, the size of Earths orbit is calculated. The Earths orbit is known with a precision of a few meters, the most important fundamental distance measurements come from trigonometric parallax. As the Earth orbits the Sun, the position of stars will appear to shift slightly against the more distant background. These shifts are angles in a triangle, with 2 AU making the base leg of the triangle. The amount of shift is small, measuring 1 arcsecond for an object at the 1 parsec distance of the nearest stars. Astronomers usually express distances in units of parsecs, light-years are used in popular media, because parallax becomes smaller for a greater stellar distance, useful distances can be measured only for stars whose parallax is larger than a few times the precision of the measurement. Parallax measurements typically have an accuracy measured in milliarcseconds, the Hubble telescope WFC3 now has the potential to provide a precision of 20 to 40 microarcseconds, enabling reliable distance measurements up to 5,000 parsecs for small numbers of stars. By the early 2020s, the GAIA space mission will provide similarly accurate distances to all bright stars. Stars have a velocity relative to the Sun that causes proper motion, for a group of stars with the same spectral class and a similar magnitude range, a mean parallax can be derived from statistical analysis of the proper motions relative to their radial velocities

9.
Parsec
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The parsec is a unit of length used to measure large distances to objects outside the Solar System. One parsec is the distance at which one astronomical unit subtends an angle of one arcsecond, a parsec is equal to about 3.26 light-years in length. The nearest star, Proxima Centauri, is about 1.3 parsecs from the Sun, most of the stars visible to the unaided eye in the nighttime sky are within 500 parsecs of the Sun. The parsec unit was likely first suggested in 1913 by the British astronomer Herbert Hall Turner, named from an abbreviation of the parallax of one arcsecond, it was defined so as to make calculations of astronomical distances quick and easy for astronomers from only their raw observational data. Partly for this reason, it is still the unit preferred in astronomy and astrophysics, though the light-year remains prominent in science texts. This corresponds to the definition of the parsec found in many contemporary astronomical references. Derivation, create a triangle with one leg being from the Earth to the Sun. As that point in space away, the angle between the Sun and Earth decreases. A parsec is the length of that leg when the angle between the Sun and Earth is one arc-second. One of the oldest methods used by astronomers to calculate the distance to a star is to record the difference in angle between two measurements of the position of the star in the sky. The first measurement is taken from the Earth on one side of the Sun, and the second is approximately half a year later. The distance between the two positions of the Earth when the two measurements were taken is twice the distance between the Earth and the Sun. The difference in angle between the two measurements is twice the angle, which is formed by lines from the Sun. Then the distance to the star could be calculated using trigonometry. 5-parsec distance of 61 Cygni, the parallax of a star is defined as half of the angular distance that a star appears to move relative to the celestial sphere as Earth orbits the Sun. Equivalently, it is the angle, from that stars perspective. The star, the Sun and the Earth form the corners of a right triangle in space, the right angle is the corner at the Sun. Therefore, given a measurement of the angle, along with the rules of trigonometry. A parsec is defined as the length of the adjacent to the vertex occupied by a star whose parallax angle is one arcsecond

10.
Apparent magnitude
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The apparent magnitude of a celestial object is a number that is a measure of its brightness as seen by an observer on Earth. The brighter an object appears, the lower its magnitude value, the Sun, at apparent magnitude of −27, is the brightest object in the sky. It is adjusted to the value it would have in the absence of the atmosphere, furthermore, the magnitude scale is logarithmic, a difference of one in magnitude corresponds to a change in brightness by a factor of 5√100, or about 2.512. The measurement of apparent magnitudes or brightnesses of celestial objects is known as photometry, apparent magnitudes are used to quantify the brightness of sources at ultraviolet, visible, and infrared wavelengths. An apparent magnitude is measured in a specific passband corresponding to some photometric system such as the UBV system. In standard astronomical notation, an apparent magnitude in the V filter band would be denoted either as mV or often simply as V, the scale used to indicate magnitude originates in the Hellenistic practice of dividing stars visible to the naked eye into six magnitudes. The brightest stars in the sky were said to be of first magnitude, whereas the faintest were of sixth magnitude. Each grade of magnitude was considered twice the brightness of the following grade and this rather crude scale for the brightness of stars was popularized by Ptolemy in his Almagest, and is generally believed to have originated with Hipparchus. This implies that a star of magnitude m is 2.512 times as bright as a star of magnitude m +1 and this figure, the fifth root of 100, became known as Pogsons Ratio. The zero point of Pogsons scale was defined by assigning Polaris a magnitude of exactly 2. However, with the advent of infrared astronomy it was revealed that Vegas radiation includes an Infrared excess presumably due to a disk consisting of dust at warm temperatures. At shorter wavelengths, there is negligible emission from dust at these temperatures, however, in order to properly extend the magnitude scale further into the infrared, this peculiarity of Vega should not affect the definition of the magnitude scale. Therefore, the scale was extrapolated to all wavelengths on the basis of the black body radiation curve for an ideal stellar surface at 11000 K uncontaminated by circumstellar radiation. On this basis the spectral irradiance for the zero magnitude point, with the modern magnitude systems, brightness over a very wide range is specified according to the logarithmic definition detailed below, using this zero reference. In practice such apparent magnitudes do not exceed 30, astronomers have developed other photometric zeropoint systems as alternatives to the Vega system. The AB magnitude zeropoint is defined such that an objects AB, the dimmer an object appears, the higher the numerical value given to its apparent magnitude, with a difference of 5 magnitudes corresponding to a brightness factor of exactly 100. Since an increase of 5 magnitudes corresponds to a decrease in brightness by a factor of exactly 100, each magnitude increase implies a decrease in brightness by the factor 5√100 ≈2.512. Inverting the above formula, a magnitude difference m1 − m2 = Δm implies a brightness factor of F2 F1 =100 Δ m 5 =100.4 Δ m ≈2.512 Δ m

11.
Galaxy morphological classification
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Galaxy morphological classification is a system used by astronomers to divide galaxies into groups based on their visual appearance. The Hubble sequence is a classification scheme for galaxies invented by Edwin Hubble in 1926. It is often known colloquially as the “Hubble tuning-fork” because of the shape in which it is traditionally represented, Hubble’s scheme divides galaxies into three broad classes based on their visual appearance, Elliptical galaxies have smooth, featureless light distributions and appear as ellipses in images. They are denoted by the letter E, followed by an integer n representing their degree of ellipticity on the sky. Spiral galaxies consist of a disk, with stars forming a spiral structure, and a central concentration of stars known as the bulge. They are given the symbol S, roughly half of all spirals are also observed to have a bar-like structure, extending from the central bulge. These barred spirals are given the symbol SB and these broad classes can be extended to enable finer distinctions of appearance and to encompass other types of galaxies, such as irregular galaxies, which have no obvious regular structure. The Hubble sequence is represented in the form of a two-pronged fork, with the ellipticals on the left. Lenticular galaxies are placed between the ellipticals and the spirals, at the point where the two meet the “handle”. To this day, the Hubble sequence is the most commonly used system for classifying galaxies, the de Vaucouleurs system for classifying galaxies is a widely used extension to the Hubble sequence, first described by Gérard de Vaucouleurs in 1959. In particular, he argued that rings and lenses are important structural components of spiral galaxies, the de Vaucouleurs system retains Hubble’s basic division of galaxies into ellipticals, lenticulars, spirals and irregulars. For example, a barred spiral galaxy with loosely wound arms. Visually, the de Vaucouleurs system can be represented as a version of Hubble’s tuning fork, with stage on the x-axis, family on the y-axis. De Vaucouleurs also assigned numerical values to each class of galaxy in his scheme, values of the numerical Hubble stage T run from −6 to +10, with negative numbers corresponding to early-type galaxies and positive numbers to late types. Elliptical galaxies are divided into three stages, compact ellipticals, normal ellipticals and late types, lenticulars are similarly subdivided into early, intermediate and late types. Irregular galaxies can be of type magellanic irregulars or compact, the use of numerical stages allows for more quantitative studies of galaxy morphology. Created by American astronomer William Wilson Morgan, together with Philip Keenan, Morgan developed the MK system for the classification of stars through their spectra. The Yerkes scheme uses the spectra of stars in the galaxy, the shape, real and apparent, thus, for example, the Andromeda Galaxy is classified as kS5. H

12.
Mass
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In physics, mass is a property of a physical body. It is the measure of a resistance to acceleration when a net force is applied. It also determines the strength of its gravitational attraction to other bodies. The basic SI unit of mass is the kilogram, Mass is not the same as weight, even though mass is often determined by measuring the objects weight using a spring scale, rather than comparing it directly with known masses. An object on the Moon would weigh less than it does on Earth because of the lower gravity and this is because weight is a force, while mass is the property that determines the strength of this force. In Newtonian physics, mass can be generalized as the amount of matter in an object, however, at very high speeds, special relativity postulates that energy is an additional source of mass. Thus, any body having mass has an equivalent amount of energy. In addition, matter is a defined term in science. There are several distinct phenomena which can be used to measure mass, active gravitational mass measures the gravitational force exerted by an object. Passive gravitational mass measures the force exerted on an object in a known gravitational field. The mass of an object determines its acceleration in the presence of an applied force, according to Newtons second law of motion, if a body of fixed mass m is subjected to a single force F, its acceleration a is given by F/m. A bodys mass also determines the degree to which it generates or is affected by a gravitational field and this is sometimes referred to as gravitational mass. The standard International System of Units unit of mass is the kilogram, the kilogram is 1000 grams, first defined in 1795 as one cubic decimeter of water at the melting point of ice. Then in 1889, the kilogram was redefined as the mass of the prototype kilogram. As of January 2013, there are proposals for redefining the kilogram yet again. In this context, the mass has units of eV/c2, the electronvolt and its multiples, such as the MeV, are commonly used in particle physics. The atomic mass unit is 1/12 of the mass of a carbon-12 atom, the atomic mass unit is convenient for expressing the masses of atoms and molecules. Outside the SI system, other units of mass include, the slug is an Imperial unit of mass, the pound is a unit of both mass and force, used mainly in the United States

13.
Solar mass
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The solar mass is a standard unit of mass in astronomy, equal to approximately 1.99 ×1030 kilograms. It is used to indicate the masses of stars, as well as clusters, nebulae. It is equal to the mass of the Sun, about two kilograms, M☉ = ×1030 kg The above mass is about 332946 times the mass of Earth. Because Earth follows an orbit around the Sun, its solar mass can be computed from the equation for the orbital period of a small body orbiting a central mass. The value he obtained differs by only 1% from the modern value, the diurnal parallax of the Sun was accurately measured during the transits of Venus in 1761 and 1769, yielding a value of 9″. From the value of the parallax, one can determine the distance to the Sun from the geometry of Earth. The first person to estimate the mass of the Sun was Isaac Newton, in his work Principia, he estimated that the ratio of the mass of Earth to the Sun was about 1/28700. Later he determined that his value was based upon a faulty value for the solar parallax and he corrected his estimated ratio to 1/169282 in the third edition of the Principia. The current value for the parallax is smaller still, yielding an estimated mass ratio of 1/332946. As a unit of measurement, the solar mass came into use before the AU, the mass of the Sun has been decreasing since the time it formed. This occurs through two processes in nearly equal amounts, first, in the Suns core, hydrogen is converted into helium through nuclear fusion, in particular the p–p chain, and this reaction converts some mass into energy in the form of gamma ray photons. Most of this energy eventually radiates away from the Sun, second, high-energy protons and electrons in the atmosphere of the Sun are ejected directly into outer space as a solar wind. The original mass of the Sun at the time it reached the main sequence remains uncertain, the early Sun had much higher mass-loss rates than at present, and it may have lost anywhere from 1–7% of its natal mass over the course of its main-sequence lifetime. The Sun gains a small amount of mass through the impact of asteroids. However, as the Sun already contains 99. 86% of the Solar Systems total mass, M☉ G / c2 ≈1.48 km M☉ G / c3 ≈4.93 μs I. -J. A Bright Young Sun Consistent with Helioseismology and Warm Temperatures on Ancient Earth and Mars

14.
Angular diameter
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The angular diameter or apparent size is an angular measurement describing how large a sphere or circle appears from a given point of view. In the vision sciences it is called the angle and in optics it is the angular aperture. The angular diameter can alternatively be thought of as the angle through which an eye or camera must rotate to look from one side of an apparent circle to the opposite side, Angular radius equals half the angular diameter. When D ≫ d, we have δ ≈ d / D, for practical use, the distinction is only significant for spherical objects that are relatively close, since the small-angle approximation holds for x ≪1, arcsin ⁡ x ≈ arctan ⁡ x ≈ x. Estimates of angular diameter may be obtained by holding the hand at right angles to an extended arm. In astronomy the sizes of objects in the sky are given in terms of their angular diameter as seen from Earth. Since these angular diameters are typically small, it is common to present them in arcseconds, an arcsecond is 1/3600th of one degree, and a radian is 180/ π degrees, so one radian equals 3600*180/ π arcseconds, which is about 206265 arcseconds. Therefore, the diameter of an object with physical diameter d at a distance D, expressed in arcseconds, is given by. These objects have a diameter of one arcsecond, an object of diameter 725. The angular diameter of the Sun, from a distance of one light-year, is 0. 03″, the angular diameter 0. 03″ of the Sun given above is approximately the same as that of a person at a distance of the diameter of the Earth. Thus the angular diameter of the Sun is about 250,000 times that of Sirius, the angular diameter of the Sun is also about 250,000 times that of Alpha Centauri A. The angular diameter of the Sun is about the same as that of the Moon, even though Pluto is physically larger than Ceres, when viewed from Earth Ceres has a much larger apparent size. While angular sizes measured in degrees are useful for larger patches of sky, we need much finer units when talking about the size of galaxies. The Moons motion across the sky can be measured in size, approximately 15 degrees every hour. A one-mile-long line painted on the face of the Moon would appear to us to be about one arc-second in length, in astronomy, it is typically difficult to directly measure the distance to an object. But the object may have a physical size and a measurable angular diameter. In that case, the angular diameter formula can be inverted to yield the Angular diameter distance to distant objects as d ≡2 D tan ⁡. In non-Euclidean space, such as our universe, the angular diameter distance is only one of several definitions of distance

15.
Messier object
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The Messier objects are a set of over 100 astronomical objects first listed by French astronomer Charles Messier in 1771. The number of objects in the lists he published reached 103, a similar list had been published in 1654 by Giovanni Hodierna, but attracted attention only recently and was probably not known to Messier. The first edition covered 45 objects numbered M1 to M45, the first such addition came from Nicolas Camille Flammarion in 1921, who added Messier 104 after finding a note Messier made in a copy of the 1781 edition of the catalogue. M105 to M107 were added by Helen Sawyer Hogg in 1947, M108 and M109 by Owen Gingerich in 1960, M102 was observed by Méchain, who communicated his notes to Messier. Méchain later concluded that this object was simply a re-observation of M101, though sources suggest that the object Méchain observed was the galaxy NGC5866. Messiers final catalogue was included in the Connaissance des Temps for 1784 and these objects are still known by their Messier number from this list. Messier lived and did his work at the Hôtel de Cluny. The list he compiled contains only objects found in the sky area he could observe and he did not observe or list objects visible only from farther south, such as the Large and Small Magellanic Clouds. A summary of the astrophysics of each Messier object can be found in the Concise Catalog of Deep-sky Objects, in early spring, astronomers sometimes gather for Messier marathons, when all of the objects can be viewed over a single night

16.
New General Catalogue
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The NGC contains 7,840 objects, known as the NGC objects. It is one of the largest comprehensive catalogues, as it includes all types of space objects and is not confined to, for example. Dreyer also published two supplements to the NGC in 1895 and 1908, known as the Index Catalogues, describing a further 5,386 astronomical objects. Objects in the sky of the southern hemisphere are catalogued somewhat less thoroughly, the Revised New General Catalogue and Index Catalogue was compiled in 2009 by Wolfgang Steinicke. The original New General Catalogue was compiled during the 1880s by John Louis Emil Dreyer using observations from William Herschel and his son John, Dreyer had already published a supplement to Herschels General Catalogue of Nebulae and Clusters, containing about 1,000 new objects. In 1886, he suggested building a second supplement to the General Catalogue and this led to the publication of the New General Catalogue in the Memoirs of the Royal Astronomical Society in 1888. Assembling the NGC was a challenge, as Dreyer had to deal with many contradicting and unclear reports, while he did check some himself, the sheer number of objects meant Dreyer had to accept them as published by others for the purpose of his compilation. Dreyer was a careful transcriber and made few errors himself, and he was very thorough in his referencing, which allowed future astronomers to review the original references and publish corrections to the original NGC. The first major update to the NGC is the Index Catalogue of Nebulae and Clusters of Stars and it serves as a supplement to the NGC, and contains an additional 5,386 objects, collectively known as the IC objects. It summarizes the discoveries of galaxies, clusters and nebulae between 1888 and 1907, most of them made possible by photography, a list of corrections to the IC was published in 1912. The Revised New Catalogue of Nonstellar Astronomical Objects was compiled by Jack W. Sulentic and William G. Tifft in the early 1970s, and was published in 1973, as an update to the NGC. However, because the update had to be completed in just three summers, it failed to incorporate several previously-published corrections to the NGC data, and even introduced new errors. NGC2000.0 is a 1988 compilation of the NGC and IC made by Roger W. Sinnott and it incorporates several corrections and errata made by astronomers over the years. However, it too ignored the original publications and favoured modern corrections, the NGC/IC Project is a collaboration formed in 1993. It aims to identify all NGC and IC objects, and collect images, the Revised New General Catalogue and Index Catalogue is a compilation made by Wolfgang Steinicke in 2009. It is considered one of the most comprehensive and authoritative treatments of the NGC, messier object Catalogue of Nebulae and Clusters of Stars The Interactive NGC Catalog Online Adventures in Deep Space, Challenging Observing Projects for Amateur Astronomers

17.
Uppsala General Catalogue
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The Uppsala General Catalogue of Galaxies is a catalogue of 12,921 galaxies visible from the northern hemisphere. It was first published in 1973, the catalogue includes essentially all galaxies north of declination -02°30 and to a limiting diameter of 1.0 arcminute or to a limiting apparent magnitude of 14.5. The primary source of data is the blue prints of the Palomar Observatory Sky Survey and it also includes galaxies smaller than 1.0 arcminute in diameter but brighter than 14.5 magnitude from the Catalogue of Galaxies and of Clusters of Galaxies. The catalogue contains descriptions of the galaxies and their areas, plus conventional system classifications. Galaxy diameters are included and the classifications and descriptions are given in such a way as to provide as accurate an account as possible of the appearance of the galaxies on the prints, the accuracy of coordinates is only what is necessary for identifications purposes. There is an addendum to the catalogue called Uppsala General Catalogue Addendum which is abbreviated as UGCA, Uppsala Uppsala Astronomical Observatory Royal Society of Sciences in Uppsala

18.
Principal Galaxies Catalogue
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The Catalogue of Principal Galaxies is an astronomical catalog published in 1989 that lists B1950 and J2000 equatorial coordinates and cross-identifications for 73,197 galaxies. It is based on the Lyon-Meudon Extragalactic Database, which was started in 1983. 40,932 coordinates have standard deviations smaller than 10″, a total of 131,601 names from the 38 most common sources are listed. The Lyon-Meudon Extragalactic Database was eventually expanded into HyperLEDA, a database of a few million galaxies, Galaxies in the original PGC catalogue are numbered with their original PGC number in HyperLEDA. Numbers have also assigned for the other galaxies, although for those galaxies not in the original PGC catalogue. PGC6240 is a lenticular galaxy in the constellation Hydrus. It is located about 106 million parsecs away from Earth, PGC39058 is a dwarf galaxy which is located approximately 14 million light years away in the constellation of Draco. It is relatively nearby, however it is obscured by a star which is in front of the galaxy. Astronomical catalogue PGC info at ESOs archive of astronomical catalogues PGC readme at Centre de Données astronomiques de Strasbourg

19.
Second Cambridge Catalogue of Radio Sources
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The Second Cambridge Catalogue of Radio Sources was published in 1955 by John R Shakeshaft and colleagues. It comprised a list of 1936 sources between declinations -38 and +83, giving their right ascension, declination, both in 1950.0 coordinates, and flux density. The observations were made with the Cambridge Interferometer, at 81.5 MHz.5, key data demonstrating this came from the then-recently commissioned Mills Cross Telescope in Australia.8 derived once confusion was taken into account. The survey was superseded by the more reliable 3C and 3CR surveys. The 3C survey also used the Cambridge Interferometer, but at 159 MHz, Shakeshaft, JR, Ryle, M, Baldwin, JE, Elsmore, B, Thomson, JH. A survey of sources between declinations −38° and +83°. Memoirs of the Royal Astronomical Society

20.
Galaxy
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A galaxy is a gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and dark matter. The word galaxy is derived from the Greek galaxias, literally milky, Galaxies range in size from dwarfs with just a few billion stars to giants with one hundred trillion stars, each orbiting its galaxys center of mass. Galaxies are categorized according to their morphology as elliptical, spiral. Many galaxies are thought to have holes at their active centers. The Milky Ways central black hole, known as Sagittarius A*, has a four million times greater than the Sun. Recent estimates of the number of galaxies in the observable universe range from 200 billion to 2 trillion or more, most of the galaxies are 1,000 to 100,000 parsecs in diameter and separated by distances on the order of millions of parsecs. The space between galaxies is filled with a gas having an average density of less than one atom per cubic meter. The majority of galaxies are organized into groups, clusters. At the largest scale, these associations are generally arranged into sheets and filaments surrounded by immense voids. In Greek mythology, Zeus places his son born by a mortal woman, the infant Heracles, on Heras breast while she is asleep so that the baby will drink her divine milk and will thus become immortal. Hera wakes up while breastfeeding and then realizes she is nursing a baby, she pushes the baby away, some of her milk spills and. In the astronomical literature, the capitalized word Galaxy is often used to refer to our galaxy, the Milky Way, to distinguish it from the other galaxies in our universe. The English term Milky Way can be traced back to a story by Chaucer c. 1380, See yonder, lo, the Galaxyë Which men clepeth the Milky Wey, For hit is whyt. However, the word Universe was later understood to mean the entirety of existence, so this expression fell into disuse and the objects instead became known as galaxies. Tens of thousands of galaxies have been catalogued, but only a few have well-established names, such as the Andromeda Galaxy, the Magellanic Clouds, the Whirlpool Galaxy, and the Sombrero Galaxy. Astronomers work with numbers from certain catalogues, such as the Messier catalogue, the NGC, the IC, the CGCG, all of the well-known galaxies appear in one or more of these catalogues but each time under a different number. For example, Messier 109 is a galaxy having the number 109 in the catalogue of Messier, but also codes NGC3992, UGC6937, CGCG 269-023, MCG +09-20-044. One of the arguments to do so is that these impressive objects deserve better than uninspired codes, for instance, Bodifee and Berger propose the informal, descriptive name Callimorphus Ursae Majoris for the well-formed barred galaxy Messier 109 in Ursa Major

21.
Andromeda (mythology)
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In Greek mythology, Andromeda is the daughter of the Aethiopian king Cepheus and his wife Cassiopeia. When Cassiopeias hubris leads her to boast that Andromeda is more beautiful than the Nereids, Andromeda is stripped and chained naked to a rock as a sacrifice to sate the monster, but is saved from death by Perseus. Her name is the Latinized form of the Greek Ἀνδρομέδα or Ἀνδρομέδη, ruler of men, from ἀνήρ, ἀνδρός man, from the Renaissance, interest revived in the original story, typically as derived from Ovids account. In Greek mythology, Andromeda was the daughter of Cepheus and Cassiopeia, king and her mother Cassiopeia boasted that her daughter was more beautiful than the Nereids, the nymph-daughters of the sea god Nereus and often seen accompanying Poseidon. To punish the queen for her arrogance, Poseidon, brother to Zeus and god of the sea, the desperate king consulted the Oracle of Apollo, who announced that no respite would be found until the king sacrificed his daughter, Andromeda, to the monster. Stripped naked, she was chained to a rock on the coast, Perseus was returning from having slain the Gorgon, Medusa. After he happened upon the chained Andromeda, he approached Cetus while invisible and he set Andromeda free, and married her in spite of her having been previously promised to her uncle Phineus. At the wedding a quarrel took place between the rivals and Phineus was turned to stone by the sight of the Gorgons head, Andromeda followed her husband, first to his native island of Serifos, where he rescued his mother Danaë, and then to Tiryns in Argos. Together, they became the ancestors of the family of the Perseidae through the line of their son Perses, Perseus and Andromeda had seven sons, Perses, Alcaeus, Heleus, Mestor, Sthenelus, Electryon, and Cynurus as well as two daughters, Autochthe and Gorgophone. Their descendants ruled Mycenae from Electryon down to Eurystheus, after whom Atreus attained the kingdom, according to this mythology, Perseus is the ancestor of the Persians. Andromeda is represented in the sky by the constellation Andromeda. Four constellations are associated with the myth, jean-Baptiste Lullys opera, Persée, also dramatizes the myth. Andromeda has been the subject of ancient and modern works of art, which typically show the moment of rescue, with Andromeda usually still chained. Examples include, one of Titians poesies, and compositions by Joachim Wtewael, Veronese, many versions by Rubens, Ingres, from the Renaissance onward the chained nude figure of Andromeda typically was the centre of interest. Rembrandts Andromeda Chained to the Rocks is unusual in showing her alone, the Italian composer Salvatore Sciarrino composed an hour-long operatic drama called Perseo e Andromeda in 2000. In 1973, a film called Perseus was made in the Soviet Union as part of the Soviet animated film collection called Legends. The 1981 film Clash of the Titans retells the story of Perseus, Andromeda, and Cassiopeia, thetis was indeed a Nereid and also the future mother of Achilles. Andromeda is depicted as being strong-willed and independent, whereas in the stories she is only mentioned as being the princess whom Perseus saves from the sea monster

22.
Spiral galaxy
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A spiral galaxy is a type of galaxy originally described by Edwin Hubble in his 1936 work The Realm of the Nebulae and, as such, forms part of the Hubble sequence. Spiral galaxies consist of a flat, rotating disk containing stars, gas and dust, and these are surrounded by a much fainter halo of stars, many of which reside in globular clusters. Spiral galaxies are named for the structures that extend from the center into the galactic disc. The spiral arms are sites of ongoing star formation and are brighter than the surrounding disc because of the young, hot OB stars that inhabit them. Roughly two-thirds of all spirals are observed to have a component in the form of a bar-like structure, extending from the central bulge. Our own Milky Way has recently confirmed to be a barred spiral. The most convincing evidence for its existence comes from a recent survey, performed by the Spitzer Space Telescope, together with irregular galaxies, spiral galaxies make up approximately 60% of galaxies in the local Universe. They are mostly found in low-density regions and are rare in the centers of galaxy clusters, Spiral arms are regions of stars that extend from the center of spiral and barred spiral galaxies. These long, thin regions resemble a spiral and thus give spiral galaxies their name, naturally, different classifications of spiral galaxies have distinct arm-structures. Sc and SBc galaxies, for instance, have very loose arms, whereas Sa, either way, spiral arms contain many young, blue stars, which make the arms so bright. A bulge is a huge, tightly packed group of stars, the term commonly refers to the central group of stars found in most spiral galaxies. Using the Hubble classification, the bulge of Sa galaxies is usually composed of Population II stars, further, the bulge of Sa and SBa galaxies tends to be large. In contrast, the bulges of Sc and SBc galaxies are much smaller and are composed of young, some bulges have similar properties to those of elliptical galaxies, others simply appear as higher density centers of disks, with properties similar to disk galaxies. Many bulges are thought to host a supermassive black hole at their centers, such black holes have never been directly observed, but many indirect proofs exist. In our own galaxy, for instance, the object called Sagittarius A* is believed to be a black hole. There is a correlation between the mass of the black hole and the velocity dispersion of the stars in the bulge. However, some stars inhabit a spheroidal halo or galactic spheroid, the orbital behaviour of these stars is disputed, but they may describe retrograde and/or highly inclined orbits, or not move in regular orbits at all. The galactic halo also contains many globular clusters, due to their irregular movement around the center of the galaxy—if they do so at all—these stars often display unusually high proper motion

23.
Milky Way
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The Milky Way is the galaxy that contains our Solar System. The descriptive milky is derived from the appearance from Earth of the galaxy – a band of light seen in the night sky formed from stars that cannot be distinguished by the naked eye. The term Milky Way is a translation of the Latin via lactea, from Earth, the Milky Way appears as a band because its disk-shaped structure is viewed from within. Galileo Galilei first resolved the band of light into individual stars with his telescope in 1610, until the early 1920s, most astronomers thought that the Milky Way contained all the stars in the Universe. Following the 1920 Great Debate between the astronomers Harlow Shapley and Heber Curtis, observations by Edwin Hubble showed that the Milky Way is just one of many galaxies, the Milky Way is a barred spiral galaxy with a diameter between 100,000 light-years and 180,000 light-years. The Milky Way is estimated to contain 100–400 billion stars, there are probably at least 100 billion planets in the Milky Way. The Solar System is located within the disk, about 26,000 light-years from the Galactic Center, on the edge of one of the spiral-shaped concentrations of gas. The stars in the inner ≈10,000 light-years form a bulge, the very center is marked by an intense radio source, named Sagittarius A*, which is likely to be a supermassive black hole. Stars and gases at a range of distances from the Galactic Center orbit at approximately 220 kilometers per second. The constant rotation speed contradicts the laws of Keplerian dynamics and suggests much of the mass of the Milky Way does not emit or absorb electromagnetic radiation. This mass has been termed dark matter, the rotational period is about 240 million years at the position of the Sun. The Milky Way as a whole is moving at a velocity of approximately 600 km per second with respect to frames of reference. The oldest stars in the Milky Way are nearly as old as the Universe itself, the Milky Way has several satellite galaxies and is part of the Local Group of galaxies, which is a component of the Virgo Supercluster, which is itself a component of the Laniakea Supercluster. The Milky Way can be seen as a band of white light some 30 degrees wide arcing across the sky. Dark regions within the band, such as the Great Rift, the area of the sky obscured by the Milky Way is called the Zone of Avoidance. The Milky Way has a low surface brightness. Its visibility can be reduced by background light such as light pollution or stray light from the Moon. The sky needs to be darker than about 20.2 magnitude per square arcsecond in order for the Milky Way to be seen and it should be visible when the limiting magnitude is approximately +5.1 or better and shows a great deal of detail at +6.1

24.
Spitzer Space Telescope
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The Spitzer Space Telescope, formerly the Space Infrared Telescope Facility, is an infrared space telescope launched in 2003. It is the fourth and final of the NASA Great Observatories program, the planned mission period was to be 2.5 years with a pre-launch expectation that the mission could extend to five or slightly more years until the onboard liquid helium supply was exhausted. This occurred on 15 May 2009, without liquid helium to cool the telescope to the very low temperatures needed to operate, most of the instruments are no longer usable. All Spitzer data, from both the primary and warm phases, are archived at the Infrared Science Archive, in keeping with NASA tradition, the telescope was renamed after its successful demonstration of operation, on 18 December 2003. Unlike most telescopes that are named after famous deceased astronomers by a board of scientists, the contest led to the telescope being named in honor of astronomer Lyman Spitzer, who had promoted the concept of space telescopes in the 1940s. Spitzer wrote a 1946 report for RAND Corporation describing the advantages of an extraterrestrial observatory, the US$720 million Spitzer was launched on 25 August 2003 at 05,35,39 UTC from Cape Canaveral SLC-17B aboard a Delta II 7920H rocket. It follows a heliocentric instead of orbit, trailing and drifting away from Earths orbit at approximately 0.1 astronomical unit per year. The primary mirror is 85 centimeters in diameter, f/12, made of beryllium and was cooled to 5.5 K, by the early 1970s, astronomers began to consider the possibility of placing an infrared telescope above the obscuring effects of Earths atmosphere. Anticipating the major results from an upcoming Explorer satellite and from the Shuttle mission, long-duration spaceflights of infrared telescopes cooled to cryogenic temperatures. Earlier infrared observations had been made by both space-based and ground-based observatories, ground-based observatories have the drawback that at infrared wavelengths or frequencies, both the Earths atmosphere and the telescope itself will radiate strongly. Additionally, the atmosphere is opaque at most infrared wavelengths and this necessitates lengthy exposure times and greatly decreases the ability to detect faint objects. It could be compared to trying to observe the stars at noon, previous space observatories were launched during the 1980s and 1990s and great advances in astronomical technology have been made since then. Most of the early concepts envisioned repeated flights aboard the NASA Space Shuttle and this approach was developed in an era when the Shuttle program was expected to support weekly flights of up to 30 days duration. A May 1983 NASA proposal described SIRTF as a Shuttle-attached mission, several flights were anticipated with a probable transition into a more extended mode of operation, possibly in association with a future space platform or space station. SIRTF would be a 1-meter class, cryogenically cooled, multi-user facility consisting of a telescope, the first flight was expected to occur about 1990, with the succeeding flights anticipated beginning approximately one year later. By September 1983 NASA was considering the possibility of a long duration SIRTF mission, Spitzer is the only one of the Great Observatories not launched by the Space Shuttle, as was originally intended. However, after the 1986 Challenger disaster, the Centaur LH2–LOX upper stage, the mission underwent a series of redesigns during the 1990s, primarily due to budget considerations. This resulted in a smaller but still fully capable mission that could use the smaller Delta II expendable launch vehicle

25.
Triangulum Galaxy
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The Triangulum Galaxy is a spiral galaxy approximately 3 million light-years from Earth in the constellation Triangulum. It is catalogued as Messier 33 or NGC598, and is informally referred to as the Pinwheel Galaxy. The Triangulum Galaxy is the third-largest member of the Local Group of galaxies, behind the Milky Way and it is one of the most distant permanent objects that can be viewed with the naked eye. It also has an H-II nucleus, the galaxy gets its name from the constellation Triangulum, which is where it can be spotted. The Triangulum Galaxy is sometimes referred to as the Pinwheel Galaxy by some amateur astronomy references. Under exceptionally good viewing conditions with no light pollution, the Triangulum Galaxy can be seen with the naked eye and it is one of the most distant permanent objects that can be viewed without the aid of a telescope. Being a diffuse object, its visibility is affected by small amounts of light pollution. It ranges from easily visible by direct vision in dark skies to a difficult averted vision object in rural or suburban skies, for this reason, Triangulum is one of the critical sky marks of the Bortle Dark-Sky Scale. The Triangulum Galaxy was probably discovered by the Italian astronomer Giovanni Battista Hodierna before 1654, in his work De systemate orbis cometici, deque admirandis coeli caracteribus, he listed it as a cloud-like nebulosity or obscuration and gave the cryptic description, near the Triangle hinc inde. This is in reference to the constellation of Triangulum as a pair of triangles, the magnitude of the object matches M33, so it is most likely a reference to the Triangulum galaxy. The galaxy was discovered by Charles Messier on the night of August 25–26,1764. It was published in his Catalog of Nebulae and Star Clusters as object number 33, when William Herschel compiled his extensive catalogue of nebulae, he was careful not to include most of the objects identified by Messier. However, M33 was an exception and he catalogued this object on September 11,1784 as H V-17, Herschel also catalogued the Triangulum Galaxys brightest and largest H II region as H III.150 separately from the galaxy itself, the nebula eventually obtained NGC number 604. As seen from Earth, NGC604 is located northeast of the central core. It is one of the largest H II regions known, with a diameter of nearly 1500 light-years, Herschel also noted 3 other smaller H II regions. It was among the first spiral nebulae identified as such by Lord Rosse in 1850, in 1922–23, John Charles Duncan and Max Wolf discovered variable stars in the nebulae. Edwin Hubble showed in 1926 that 35 of these stars were classical Cepheids, the results were consistent with the concept of spiral nebulae being independent galactic systems of gas and dust, rather than just nebulae in the Milky Way. With a diameter of about 60,000 light-years, the Triangulum galaxy is the third largest member of the Local Group of galaxies and it may be a gravitationally bound companion of the Andromeda Galaxy

26.
Local Group
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The Local Group is the galaxy group that includes the Milky Way. The Local Group comprises more than 54 galaxies, most of them dwarf galaxies and its gravitational center is located somewhere between the Milky Way and the Andromeda Galaxy. The Local Group covers a diameter of 10 Mly and has a binary distribution, the group itself is a part of the larger Virgo Supercluster, which in turn may be a part of the Laniakea Supercluster. The three largest members of the group are the Andromeda Galaxy, the Milky Way and the Triangulum Galaxy, the larger two of these spiral galaxies each have their own system of satellite galaxies. The Triangulum Galaxy may or may not be a companion to the Andromeda Galaxy, pisces Dwarf is equidistant from the Andromeda Galaxy and the Triangulum Galaxy, so it may be a satellite of either. The membership of NGC3109, with its companions Sextans A, the term The Local Group was introduced by Edwin Hubble in Chapter VI of his 1936 book The Realm of the Nebulae. He also identified IC10 as a possible Local Group member, by 2003, the number of known Local Group members had increased from his initial 12 to 36. Smiths Cloud, a high-velocity cloud, between 32,000 and 49,000 light years from Earth and 8,000 light years from the disk of the Milky Way galaxy HVC 127-41-330, updated Information on the Local Group. The Publications of the Astronomical Society of the Pacific

27.
Elliptical galaxy
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An elliptical galaxy is a type of galaxy having an approximately ellipsoidal shape and a smooth, nearly featureless brightness profile. Unlike flat spiral galaxies with organization and structure, they are more three-dimensional, without much structure and they are one of the three main classes of galaxy originally described by Edwin Hubble in his 1936 work The Realm of the Nebulae, along with spiral and lenticular galaxies. Elliptical galaxies range in shape from spherical to highly flat. Originally Edwin Hubble hypothesized that elliptical galaxies evolved into spiral galaxies, stars found inside of elliptical galaxies are much older than stars found in spiral galaxies. Elliptical galaxies are believed to make up approximately 10%–15% of galaxies in the Virgo Supercluster and they are preferentially found close to the centers of galaxy clusters. Elliptical galaxies are also called early-type galaxies, due to their location in the Hubble sequence, elliptical galaxies are characterized by several properties that make them distinct from other classes of galaxy. They are spherical or ovoid masses of stars, starved of star-making gases, the smallest known elliptical galaxy is about one-tenth the size of the Milky Way. The motion of stars in galaxies is predominantly radial, unlike the disks of spiral galaxies. Large elliptical galaxies typically have a system of globular clusters. The dynamical properties of galaxies and the bulges of disk galaxies are similar, suggesting that they may be formed by the same physical processes. The luminosity profiles of both elliptical galaxies and bulges are well fit by Sersics law, every massive elliptical galaxy contains a supermassive black hole at its center. Observations of 46 elliptical galaxies,20 classical bulges, and 22 pseudobulges show that each contain a black hole at the center, elliptical galaxies are preferentially found in galaxy clusters and in compact groups of galaxies. The traditional portrait of elliptical galaxies paints them as galaxies where star formation finished after an initial burst at high-redshift, elliptical galaxies typically appear yellow-red, which is in contrast to the distinct blue tinge of most spiral galaxies. In spirals, this blue color emanates largely from the young, very little star formation is thought to occur in elliptical galaxies, because of their lack of gas compared to spiral or irregular galaxies. However, in recent years, evidence has shown that a proportion of these galaxies have residual gas reservoirs. Researchers with the Herschel Space Observatory have speculated that the black holes in elliptical galaxies keep the gas from cooling enough for star formation. Elliptical galaxies vary greatly in size and mass with diameters ranging from 3000 lightyears to more than 700,000 lightyears. This range is broader for this galaxy type than for any other

28.
Naked eye
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Naked eye, also called bare eye or unaided eye, is the practice of engaging in visual perception unaided by a magnifying or light-collecting optical device, such as a telescope or microscope. Vision corrected to normal acuity using corrective lenses is considered naked, in astronomy, the naked eye may be used to observe events that can be viewed without equipment, such as an astronomical conjunction, the passage of a comet, or a meteor shower. Sky lore and various tests demonstrate a wealth of phenomena that can be seen with the unaided eye. The basic accuracies of the eye are, Quick autofocus from distances of 25 cm to 50 cm to infinity. Angular resolution, about 1 arcminute, approximately 0. 02° or 0.0003 radians, field of view, simultaneous visual perception in an area of about 160° × 175°. Faint stars up to +8 magnitude under a dark sky. Photometry to ±10% or 1% of intensity – in a range between night and day of 1,10,000,000,000, symmetries of 10–20, see the measurements of Tycho Brahe and the Egyptians. Even a few hundred kilometers away from an area where the sky can appear to be very dark. For most people, these are likely to be the best observing conditions within their reach, under such typical dark sky conditions, the naked eye can see stars with an apparent magnitude up to +6m. Under perfect dark sky conditions where all light pollution is absent, the angular resolution of the naked eye is about 1′, however, some people have sharper vision than that. There is anecdotal evidence that people had seen the Galilean moons of Jupiter before telescopes were invented, Uranus, when discovered in 1781, was the first planet discovered using technology rather than being spotted by the naked eye. In practice, the extinction and dust reduces this number somewhat. In the center of a city, where the limiting magnitude due to extreme amounts of light pollution can be +4m or less. Colors can be seen but this is limited by the fact that the eye uses rods instead of cones to view fainter stars, the visibility of diffuse objects such as star clusters and galaxies is much more strongly affected by light pollution than is that of planets and stars. Under typical dark conditions only a few objects are visible. The Triangulum Galaxy is a difficult averted vision object and only visible at all if it is higher than 50° in the sky, the globular clusters M3 in Canes Venatici and M92 in Hercules are also visible with the naked eye under such conditions. Under really dark sky conditions, however, M33 is easy to see, many other Messier objects are also visible under such conditions. The most distant objects that have seen by the naked eye are nearby bright galaxies such as Centaurus A, Bodes Galaxy, Sculptor Galaxy

29.
Light pollution
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Light pollution, also known as photopollution, is excessive, misdirected or obtrusive artificial light. As a major side-effect of urbanization, it is blamed for compromising health, disrupting ecosystems, Light pollution is the adding-of/added light itself, in analogy to added sound, carbon dioxide, etc. Adverse consequences are multiple, some of them may not be known yet, scientific definitions thus include the following, The degradation of photic habitat by artificial light. The alteration of light levels in the outdoor environment owing to artificial light sources. The alteration of light levels in the environment due to man-made sources of light. Indoor light pollution is such alteration of light levels in the environment due to sources of light. The introduction by humans, directly or indirectly, of light into the environment. The first three of the four scientific definitions describe the state of the environment. The fourth one describes the process of polluting by light, Light pollution is a side effect of industrial civilization. Its sources include building exterior and interior lighting, advertising, commercial properties, offices, factories, streetlights, since the early 1980s, a global dark-sky movement has emerged, with concerned people campaigning to reduce the amount of light pollution. The International Dark-Sky Association is one non-profit advocacy group involved in this movement, several industry groups also recognize light pollution as an important issue. For example, the Institution of Lighting Engineers in the United Kingdom provides its members with information about light pollution, the problems it causes, and how to reduce its impact. Since not everyone is irritated by the same lighting sources, it is common for one persons light pollution to be light that is desirable for another. One example of this is found in advertising, when an advertiser wishes for particular lights to be bright and visible, other types of light pollution are more certain. For instance, light that crosses a property boundary and annoys a neighbor is generally wasted. Where objective measurement is desired, light levels can be quantified by field measurement or mathematical modeling, authorities have also taken a variety of measures for dealing with light pollution, depending on the interests, beliefs and understandings of the society involved. Measures range from doing nothing at all, to implementing strict laws, Light pollution is a broad term that refers to multiple problems, all of which are caused by inefficient, unappealing, or unnecessary use of artificial light. Specific categories of light pollution include light trespass, over-illumination, glare, light clutter, a single offending light source often falls into more than one of these categories

30.
Isaac Roberts
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Isaac Roberts was a Welsh engineer and business man best known for his work as an amateur astronomer, pioneering the field of astrophotography of nebulae. He was a member of the Liverpool Astronomical Society in England and was a fellow of the Royal Geological Society, Roberts was also awarded the Gold Medal of the Royal Astronomical Society in 1895. Roberts was born in Groes, Denbighshire, Wales to William Roberts, although he spent some years of his childhood there, he later moved to Liverpool. There, he became an apprentice to John Johnson & Son and he became a partner in 1847, and supplemented his job with night school. When Peter Robinson died in 1855, Roberts was made manager of the firm, when the other partner, John Johnson died, Roberts was in charge of the contracts and affairs of the firm. Roberts began working as a builder in 1859, and was joined by Peter Robinsons son, J. J. Robinson and he was very successful, and became known as one of the best engineers in the region. Isaac Roberts married his first wife, Ellen Anne, in 1875, Isaac Roberts married Dorothea Klumpke, who was nearly 30 years his junior, in 1901. He became agnostic in his religious views, Roberts died suddenly in Crowborough, Sussex, England in 1904, widowing his then-wife Dorethea Klumpke. He was cremated soon after his death, and his ashes lay in Crowborough for about five years before he was reburied in Flaybrick Hill Cemetery, Roberts was patriotic to his home land of Wales, and continued to use the Welsh language throughout his life. He left an amount of money to Cardiff University, Bangor University. His epitaph read, In memory of Isaac Roberts, Fellow of the Royal Society, one of Englands pioneers in the domain of Celestial Photography. Born at Groes, near Denbigh,27 January 1829, died at Starfield, Crowboro, Sussex,17 July 1904, who spent his life in the search after Truth. This stone is erected in loving devotion by his widow Dorethea Roberts née Klumpke, the crater Roberts on the far side of the Moon was named to jointly honour Isaac Roberts and the South African astronomer Alexander William Roberts. In 1878, Roberts had a 7-inch refractor at his home in Rock Ferry, although at the time he used this for visual observation, he began to explore stellar photography, his forte, a few years later. In 1883, Roberts began experimenting with astrophotography and he first used portrait lenses with apertures varying from 38 to 8 inches. He mounted photographic plates directly at the focus in order to avoid the loss of light that would occur from using a second mirror. This allowed him to significant progress in the then-developing field of astrophotography. In 1886 Roberts displayed his first photographs at the Royal Astronomical Society at Liverpool and these images showed, for the first time, the vast extensions of nebulosity in the Pleiades and Orion

31.
Persian Empire
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Persian Empire refers to any of a series of imperial dynasties centered in Persia. The first of these was the Achaemenid Empire established by Cyrus the Great in 550 BC with the conquest of Median, Lydian and Babylonian empires and it covered much of the Ancient world when it was conquered by Alexander the Great. Several later dynasties claimed to be heirs of the Achaemenids, Persia was then ruled by the Parthian Empire which supplanted the Hellenistic Seleucid Empire, and then by the Sassanian Empire which ruled up until mid 7th century. It is important to note that many of these empires referred to themselves as Persian, they were often ethnically ruled by Medes, Babylonians. Iranian dynastic history was interrupted by the Arab Muslim conquest of Persia in 651 AD, establishing the even larger Islamic Caliphate, the main religion of ancient Persia was the native Zoroastrianism, but after the seventh century, it was replaced by Islam. Since 1979 and the downfall of the Pahlavi dynasty during Iranian Revolution, Persia has had a Shiah theocratic government

32.
Abd al-Rahman al-Sufi
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The lunar crater Azophi and the minor planet 12621 Alsufi are named after him. Al-Sufi published his famous Book of Fixed Stars in 964, describing much of his work, al-Biruni reports that his work on the ecliptic was carried out in Shiraz. He lived at the Buyid court in Isfahan, abd al-Rahman al-Sufi was one of the famous nine Muslim astronomers. His name implies that he was from a Sufi Muslim background and he lived at the court of Emir Adud ad-Daula in Isfahan, Persia, and worked on translating and expanding Greek astronomical works, especially the Almagest of Ptolemy. He contributed several corrections to Ptolemys star list and did his own brightness and he identified the Large Magellanic Cloud, which is visible from Yemen, though not from Isfahan, it was not seen by Europeans until Magellans voyage in the 16th century. He also made the earliest recorded observation of the Andromeda Galaxy in 964 AD and these were the first galaxies other than the Milky Way to be observed from Earth. He observed that the plane is inclined with respect to the celestial equator. He observed and described the stars, their positions, their magnitudes and their colour, for each constellation, he provided two drawings, one from the outside of a celestial globe, and the other from the inside. Since 2006, Astronomy Society of Iran – Amateur Committee hold an international Sufi Observing Competition in the memory of Sufi, the first competition was held in 2006 in the north of Semnan Province and the second was held in the summer of 2008 in Ladiz near the Zahedan. More than 100 attendees from Iran and Iraq participated in the event

33.
Book of Fixed Stars
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The Book of Fixed Stars is an astronomical text written by Abd al-Rahman al-Sufi around 964. The book was written in Arabic, although the author himself was Persian and it was an attempt to create a synthesis of the comprehensive star catalogue in Ptolemy’s Almagest with the indigenous Arabic astronomical traditions on the constellations. The book was thoroughly illustrated along with observations and descriptions of the stars, their positions, their magnitudes and his results, as in Ptolemys Almagest, were set out constellation by constellation. For each constellation, he provided two drawings, one from the outside of a globe, and the other from the inside. The work was influential and survives in numerous manuscripts and translations. The oldest manuscript, kept in the Bodleian Library, dates to 1009 and is the work of the authors son, there is a thirteenth-century copy in the British Library. He has the earliest known descriptions and illustrations of what he called a little cloud and he mentions it as lying before the mouth of a Big Fish, an Arabic constellation. This cloud was apparently known to the Isfahan astronomers, very probably before 905. The first recorded mention of the Large Magellanic Cloud was also given in the Book of Fixed Stars and these were the first galaxies other than the Milky Way to be observed from Earth. The Great Andromeda Nebula he observed was also the first true nebula to be observed, moreover, he mentions the two Magellanic Clouds, and that they are not visible from Iraq nor Najd, but visible from Tihama, and that they are called al-Baqar. There has not been a published English translation of the book, as of March 2012, one is in preparation by Ihsan Hafez of James Cook University, Townsville. Text and French translation of introduction by J. J. A. Caussin de Perceval in Notices et extraits des manuscrits XII, Paris,1831. Schjellerup, Description des étoiles fixes par Abd-al-Rahman al-Sûfi, St. Petersburg,1874, complete French translation from two late mss. with selected portions in Arabic. Ketāb ṣowar al-kawākeb al-ṯābeta, edited from five mss. and accompanied by the Orǰūza of Ebn al-Ṣūfī, Hyderabad, facsimile edition of the Persian translation by Naṣīr-al-dīn Ṭūsī, Tehran,1348 Š. /1969. Critical edition of Ṭūsīs translation by Sayyed Moʿezz-al-dīn Mahdavī, Tehran,1351 Š. /1972, the Italian translation was edited by P. Knecht, I libri astronomici di Alfonso X in una versione fiorentina del trecento, Saragossa,1965. Paul Kunitzsch, The Arabs and the Stars, Texts and Traditions on the Fixed Stars, Paul Kunitzsch, Ṣūfī Latinus, Zeitschrift der Deutschen Morgenländische Gesellschaft,115,1965, pp. 65–74. Paul Kunitzsch, Al-Ṣūfī in, Dictionary of Scientific Biography, XIII, New York,1976, J. Upton, A Manuscript of “The Book of the Fixed Stars” by ʿAbd ar-Raḥmān aṣ-Ṣūfī, Metropolitan Museum Studies,4,1933, pp. 179–97. E. Wellesz, An Islamic Book of Constellations, Oxford,1965, H. J. J. Winter, Notes on al-Kitab Suwar Al-Kawakib, Archives Internationales d’Histoire des Sciences,8,1955, pp. 126–33

34.
Star chart
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A star chart is another name for a chore chart. A star chart or star map is a map of the night sky, astronomers divide these into grids to use them more easily. They are used to identify and locate objects such as stars, constellations. They have been used for navigation since time immemorial. Note that a star chart differs from a catalog, which is a listing or tabulation of astronomical objects for a particular purpose. Tools utilizing a star chart include the astrolabe and the planisphere, the oldest known star chart may be a carved ivory Mammoth tusk that was discovered in Germany in 1979. This artifact is 32,500 years old and has a carving that resembles the constellation Orion, a drawing on the wall of the Lascaux caves in France has a graphical representation of the Pleiades open cluster of stars. This is dated from 33,000 to 10,000 years ago, another star chart panel, created more than 21,000 years ago, was found in the La Tête du Lion grotto. The bovine in this panel may represent the constellation Taurus, with a representing the Pleiades just above it. The oldest accurately dated star chart appeared in ancient Egyptian astronomy in 1534 BC, the earliest known star catalogues were compiled by the ancient Babylonian astronomers of Mesopotamia in the late 2nd millennium BC, during the Kassite Period. The oldest Chinese astronomy records date to before the Warring States period, the oldest Chinese graphical representation of the sky is a lacquer box dated to 430 BC, although this depiction does not show individual stars. The Farnese Atlas is a 2nd-century copy of a Hellenistic era statue depicting the Titan Atlas holding the sphere on his shoulder. It is the oldest surviving depiction of the ancient Greek constellations, because of precession, the positions of the constellations slowly change over time. By comparing the positions of the 41 constellations against the grid circles, based upon this information, the constellations were catalogued at 125 ±55 BC. This evidence indicates that the catalogue of the Greek astronomer Hipparchus was used. A Roman era example of a representation of the night sky is the Egyptian Dendera zodiac. This is a bas relief sculpting on a ceiling at the Dendera Temple complex and it is a planisphere depicting the zodiac in graphical representations. However, individual stars are not plotted, the oldest surviving manuscript star chart was discovered in the Mogao Caves along the Silk Road – the Dunhuang Star Chart

35.
Simon Marius
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Simon Marius was a German astronomer. He was born in Gunzenhausen, near Nuremberg, but he spent most of his life in the city of Ansbach, in 1614 Marius published his work Mundus Iovialis describing the planet Jupiter and its moons. Here he claimed to have discovered the four major moons some days before Galileo Galilei. This led to a dispute with Galileo, who in Il Saggiatore in 1623 accused Marius of plagiarism, but a jury in The Netherlands in 2003 examined the evidence extensively and ruled in favor of Mariuss independent discoveries, with results published by Bosscha in 1907. Regardless of priority, the names by which these satellites are known today are those given them by Marius, Io, Europa, Ganimedes puer. Io, Europa, the boy Ganymede, and Callisto greatly pleased lustful Jupiter, Simon Marius also observed the Andromeda nebula, which had also been known to Arab astronomers of the Middle Ages. That he detected the spurious disks of stars created by his telescope and that, from his observations of the Jovian moons he derived better periods of revolution and other orbital elements for them than did Galileo. That he observed the location of Tycho Brahes supernova of 1572, Marius drew conclusions about the structure of the universe from his observations of the Jovian moons and the stellar disks. The stellar disks he observed were spurious, but Marius interpreted them to be physical disks and he also concluded from his observations of the Jovian moons that they must orbit Jupiter while Jupiter orbits the Sun. Therefore, Marius concluded that the geocentric Tychonic system, in which the circle the Sun while the Sun circles the Earth, must be the correct world system. Mundus Iovialis anno MDCIX Detectus Ope Perspicilli Belgici,1614 Zinner, E. Zur Ehrenrettung des Simon Marius, in, heft, Leipzig 1942 Bosscha, J. Simon Marius. Réhabilitation d´un astronome calomnié, in, Archives Nederlandaises des Sciences Exactes et Naturelles, II, T. XII, pp. 258–307, 490–528, La Haye,1907 Marius-Portal — Mathematician – Medical Practitioner – Astronomer. The Galileo Project — biography of Simon Marius, Simon-Marius-Gymnasium — Simon-Marius-Gymnasium Gunzenhausen, named after the astronomer. Online Galleries, History of Science Collections, University of Oklahoma Libraries High resolution images of works by and/or portraits of Simon Marius in. jpg, oConnor, John J. Robertson, Edmund F. Simon Marius, MacTutor History of Mathematics archive, University of St Andrews

36.
Immanuel Kant
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Immanuel Kant was a German philosopher who is considered a central figure in modern philosophy. Kant took himself to have effected a Copernican revolution in philosophy and his beliefs continue to have a major influence on contemporary philosophy, especially the fields of metaphysics, epistemology, ethics, political theory, and aesthetics. Politically, Kant was one of the earliest exponents of the idea that peace could be secured through universal democracy. He believed that this will be the outcome of universal history. Kant wanted to put an end to an era of futile and speculative theories of human experience, Kant argued that our experiences are structured by necessary features of our minds. In his view, the shapes and structures experience so that, on an abstract level. Among other things, Kant believed that the concepts of space and time are integral to all human experience, as are our concepts of cause, Kant published other important works on ethics, religion, law, aesthetics, astronomy, and history. These included the Critique of Practical Reason, the Metaphysics of Morals, which dealt with ethics, and the Critique of Judgment, Immanuel Kant was born in 1724 in Königsberg, Prussia. His mother, Anna Regina Reuter, was born in Königsberg to a father from Nuremberg. His father, Johann Georg Kant, was a German harness maker from Memel, Immanuel Kant believed that his paternal grandfather Hans Kant was of Scottish origin. Kant was the fourth of nine children, baptized Emanuel, he changed his name to Immanuel after learning Hebrew. Young Kant was a solid, albeit unspectacular, student and he was brought up in a Pietist household that stressed religious devotion, humility, and a literal interpretation of the Bible. His education was strict, punitive and disciplinary, and focused on Latin and religious instruction over mathematics, despite his religious upbringing and maintaining a belief in God, Kant was skeptical of religion in later life, various commentators have labelled him agnostic. Common myths about Kants personal mannerisms are listed, explained, and refuted in Goldthwaits introduction to his translation of Observations on the Feeling of the Beautiful and Sublime. It is often held that Kant lived a strict and disciplined life. He never married, but seemed to have a social life — he was a popular teacher. He had a circle of friends whom he met, among them Joseph Green. A common myth is that Kant never traveled more than 16 kilometres from Königsberg his whole life, in fact, between 1750 and 1754 he worked as a tutor in Judtschen and in Groß-Arnsdorf

37.
Charles Messier
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Charles Messier was a French astronomer most notable for publishing an astronomical catalogue consisting of nebulae and star clusters that came to be known as the 110 Messier objects. The purpose of the catalogue was to help astronomical observers, in particular comet hunters such as himself, Messier was born in Badonviller in the Lorraine region of France, being the tenth of twelve children of Françoise B. Grandblaise and Nicolas Messier, a Court usher, six of his brothers and sisters died while young and in 1741, his father died. Charles interest in astronomy was stimulated by the appearance of the spectacular, great six-tailed comet in 1744, in 1751 he entered the employ of Joseph Nicolas Delisle, the astronomer of the French Navy, who instructed him to keep careful records of his observations. Messiers first documented observation was that of the Mercury transit of 6 May 1753, according to Meyer, Messier is buried in Père Lachaise Cemetery, Paris, in Section 11. Messiers occupation as a comet hunter led him to come across fixed diffuse objects in the night sky which could be mistaken for comets. He compiled a list of them, in collaboration with his friend and assistant Pierre Méchain, the entries are now known to be galaxies, planetary nebulae, other types of nebulae, and star clusters. Messier did his observing with a 100 mm refracting telescope from Hôtel de Cluny, in downtown Paris, the list he compiled contains only objects found in the area of the sky he could observe, from the north celestial pole to a declination of about −35. 7°. They are not organized scientifically by object type, or even by location, the first version of Messiers catalogue contained 45 objects and was published in 1774 in the journal of the French Academy of Sciences in Paris. In addition to his own discoveries, this version included objects previously observed by other astronomers, by 1780 the catalog had increased to 80 objects. The final version of the catalogue was published in 1781, in the 1784 issue of Connaissance des Temps, the final list of Messier objects had grown to 103. These seven objects, M104 through M110, are accepted by astronomers as official Messier objects, the crater Messier on the Moon and the asteroid 7359 Messier were named in his honor. Deep sky object List of Messier objects Messier object Messier marathon Caldwell catalogue OMeara, deep Sky Companions, The Messier Objects. Charles Messier Biography at Students for the Exploration and Development of Space, retrieved July 2007 Short biography of Charles Messier and history of the Messier Object Catalog by Jon Zander at OurDarkSkies. com. Retrieved July 2007 Life of a Comet Hunter, Messier and Astrobiology Professor Mark Brake and Martin Griffiths, Astrobiology Magazine European Edition, retrieved July 2007 Interactive Messier Catalog Greenhawk Observatory Amateur Photos of Charles Messier Objects Biography - Messier website. Messier Marathon Attempts to find as many Messier objects as possible in one night, retrieved July 2007 Clickable table of Messier objects Charles Messier explains his catalog on YouTube

38.
William Herschel
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Frederick William Herschel, KH, FRS was a British astronomer and composer of German origin, and brother of fellow astronomer Caroline Herschel, with whom he worked. Born in the Electorate of Hanover, Herschel followed his father into the Military Band of Hanover, Herschel constructed his first large telescope in 1774, after which he spent nine years carrying out sky surveys to investigate double stars. The resolving power of the Herschel telescopes revealed that the nebulae in the Messier catalogue were clusters of stars, Herschel published catalogues of nebulae in 1802 and in 1820. In the course of an observation on 13 March 1781, he realized that one celestial body he had observed was not a star and this was the first planet to be discovered since antiquity and Herschel became famous overnight. As a result of this discovery, George III appointed him Court Astronomer and he was elected as a Fellow of the Royal Society and grants were provided for the construction of new telescopes. Herschel pioneered the use of astronomical spectrophotometry as a tool, using prisms. Other work included a determination of the rotation period of Mars, the discovery that the Martian polar caps vary seasonally. In addition, Herschel discovered infrared radiation, Herschel was made a Knight of the Royal Guelphic Order in 1816. He was the first President of the Royal Astronomical Society when it was founded in 1820 and he died in August 1822, and his work was continued by his only son, John Herschel. Herschel was born in the Electorate of Hanover in Germany, then part of the Holy Roman Empire and his father was an oboist in the Hanover Military Band. In 1755 the Hanoverian Guards regiment, in whose band Wilhelm, at the time the crowns of Great Britain and Hanover were united under King George II. As the threat of war with France loomed, the Hanoverian Guards were recalled from England to defend Hanover, after they were defeated at the Battle of Hastenbeck, Herschels father Isaak sent his two sons to seek refuge in England in late 1757. Although his older brother Jakob had received his dismissal from the Hanoverian Guards, Wilhelm, nineteen years old at this time, was a quick student of the English language. In England he went by the English rendition of his name, in addition to the oboe, he played the violin and harpsichord and later the organ. He composed numerous works, including 24 symphonies and many concertos. Six of his symphonies were recorded in April 2002 by the London Mozart Players, Herschel moved to Sunderland in 1761 when Charles Avison immediately engaged him as first violin and soloist for his Newcastle orchestra, where he played for one season. In ‘Sunderland in the County of Durh, apprill 20th 1761’ he wrote his Symphony No.8 in C Minor. He was head of the Durham Militia band 1760–61 and visited the home of Sir Ralph Milbanke at Halnaby Hall in 1760, after Newcastle he moved to Leeds and Halifax where he was the first organist at St John the Baptist church

39.
Sirius
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Sirius is a star system and the brightest star in the Earths night sky. With a visual apparent magnitude of −1.46, it is almost twice as bright as Canopus, the system has the Bayer designation Alpha Canis Majoris. The distance separating Sirius A from its companion varies between 8.2 and 31.5 AU, Sirius appears bright because of its intrinsic luminosity and its proximity to Earth. At a distance of 2.6 parsecs, as determined by the Hipparcos astrometry satellite, Sirius is gradually moving closer to the Solar System, so it will slightly increase in brightness over the next 60,000 years. After that time its distance will begin to increase and it will become fainter, Sirius A is about twice as massive as the Sun and has an absolute visual magnitude of 1.42. It is 25 times more luminous than the Sun but has a lower luminosity than other bright stars such as Canopus or Rigel. The system is between 200 and 300 million years old and it was originally composed of two bright bluish stars. Sirius is also known colloquially as the Dog Star, reflecting its prominence in its constellation, the brightest star in the night sky, Sirius is recorded in the earliest astronomical records. Every year, it disappears for seventy days before returning to the sky just before sunrise. This occurs at Cairo on 19 July, placing it just prior to the summer solstice, the ancient Greeks observed that the appearance of Sirius heralded the hot and dry summer and feared that it caused plants to wilt, men to weaken, and women to become aroused. Due to its brightness, Sirius would have been noted to twinkle more in the weather conditions of early summer. To Greek observers, this signified certain emanations which caused its malignant influence, anyone suffering its effects was said to be star-struck. It was described as burning or flaming in literature, the season following the stars reappearance came to be known as the dog days. The inhabitants of the island of Ceos in the Aegean Sea would offer sacrifices to Sirius and Zeus to bring cooling breezes, if it rose clear, it would portend good fortune, if it was misty or faint then it foretold pestilence. Coins retrieved from the island from the 3rd century BC feature dogs or stars with emanating rays, ptolemy of Alexandria mapped the stars in Books VII and VIII of his Almagest, in which he used Sirius as the location for the globes central meridian. He depicted it as one of six red-coloured stars, the other five are class M and K stars, such as Arcturus and Betelgeuse. Bright stars were important to the ancient Polynesians for navigation between the islands and atolls of the Pacific Ocean. Low on the horizon, they acted as stellar compasses and they also served as latitude markers, the declination of Sirius matches the latitude of the archipelago of Fiji at 17°S and thus passes directly over the islands each night

40.
William Parsons, 3rd Earl of Rosse
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William Parsons, 3rd Earl of Rosse KP PRS was an Anglo-Irish astronomer who had several telescopes built. His 72-inch telescope, built in 1845 and colloquially known as the Leviathan of Parsonstown, was the worlds largest telescope, in terms of aperture size, from 1807 until 1841, he was styled as Baron Oxmantown. He was born in Yorkshire, England, in the city of York and he was educated, as Lord Oxmantown, at Trinity College, Dublin, and Oxford Universitys Magdalen College, graduating with first-class honours in mathematics in 1822. He inherited an earldom and an estate in Kings County in Ireland when his father, Lawrence, 2nd Earl of Rosse. Lord Rosse married Mary Field, daughter of John Wilmer Field and they had a total of thirteen children, but only four sons survived to adulthood, Lawrence, 4th Earl of Rosse. The Hon. Richard Clere Parsons, apparently known for developing railways in South America, sir Charles Algernon Parsons, known for inventing the steam turbine. During the 1840s, he had the Leviathan of Parsonstown built, the 72-inch telescope replaced a 36-inch telescope that he had built previously. Details of the metal, casting, grinding and polishing of the 3-ton speculum were presented in 1844 at the Belfast Natural History Society, Rosses telescope was considered a marvellous technical and architectural achievement, and images of it were circulated widely within the British commonwealth. Building of the Leviathan began in 1842 and it was first used in 1845 and it was the worlds largest telescope, in terms of aperture size, until the early 20th century. Using this telescope Rosse saw and catalogued a number of nebulae. Lord Rosse performed astronomical studies and discovered the nature of some nebulas. Rosses telescope Leviathan was the first to reveal the structure of M51, a galaxy nicknamed later as the Whirlpool Galaxy. Rosse named the Crab Nebula, based on a drawing made with his older 36-inch telescope in which it resembled a crab. A few years later, when the 72-inch telescope was in service, he produced a drawing of considerably different appearance. A main component of Rosses nebular research was his attempt to resolve the nebular hypothesis, Rosse himself did not believe that nebulas were truly gaseous, arguing rather that they were made of such an amount of fine stars that most telescopes could not resolve them individually. Rosses primary opponent in this was John Herschel, who used his own instruments to claim that the Orion nebula was a true nebula, eventually, neither man could establish sufficiently scientific results to resolve the question. But the genius displayed in all the contrivances for wielding this mighty monster even surpasses the design, the telescope weighs sixteen tons, and yet Lord Rosse raised it single-handed off its resting place, and two men with ease raised it to any height. Lord Rosse had a variety of optical reflecting telescopes built, Rosses telescopes used cast speculum metal ground parabolically and polished

41.
William Huggins
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Sir William Huggins OM KCB PRS was an English astronomer best known for his pioneering work in astronomical spectroscopy together with his wife Margaret Lindsay Huggins. William Huggins was born at Cornhill, Middlesex, in 1824, in 1875 he married Margaret Lindsay, daughter of John Murray of Dublin, who also had an interest in astronomy and scientific research. She encouraged her husbands photography and helped to put their research on a systematic footing, on 29 August 1864, Huggins was the first to take the spectrum of a planetary nebula when he analysed NGC6543. Huggins was assisted in the analysis of spectra by his neighbour, Huggins was also the first to adopt dry plate photography in imaging astronomical objects. With observations of Sirius showing a redshift in 1868, Huggins hypothesized that a velocity of the star could be computed. Huggins won the Gold Medal of the Royal Astronomical Society in 1867 and he later served as President of the Royal Astronomical Society from 1876 to 1878, and received the Gold Medal again in 1885. He served as an officer of the Royal Astronomical Society for a total of 37 years, Huggins was elected a Fellow of the Royal Society in June 1865, was awarded their Royal Medal, Rumford Medal and Copley Medal and delivered their Bakerian Lecture in 1885. He then served as President of the Royal Society from 1900 to 1905, for example, his Presidential Address in 1904 praised the fallen Fellows and distributed the prizes of that year. He died at his home in Tulse Hill, London, after an operation for a hernia in 1910 and was buried at Golders Green Cemetery, London,1906, The Royal Society, or, Science in the state and in the schools. 1909, The Scientific Papers of Sir William Huggins, edited by Sir William and Lady Huggins. London, Planetary nebula#Observations Works written by or about William Huggins at Wikisource Huggins, Sir William Barbara J. Becker William Wallace Campbell Sir William Huggins, Astronomical Society of the Pacific link from Internet Archive

42.
Spectrum
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A spectrum is a condition that is not limited to a specific set of values but can vary, without steps, across a continuum. The word was first used scientifically in optics to describe the rainbow of colors in visible light after passing through a prism, as scientific understanding of light advanced, it came to apply to the entire electromagnetic spectrum. Spectrum has since been applied by analogy to topics outside of optics, thus, one might talk about the spectrum of political opinion, or the spectrum of activity of a drug, or the autism spectrum. In these uses, values within a spectrum may not be associated with precisely quantifiable numbers or definitions, such uses imply a broad range of conditions or behaviors grouped together and studied under a single title for ease of discussion. Nonscientific uses of the spectrum are sometimes misleading. For instance, a single left–right spectrum of opinion does not capture the full range of peoples political beliefs. Political scientists use a variety of biaxial and multiaxial systems to accurately characterize political opinion. In most modern usages of spectrum there is a theme between the extremes at either end. This was not always true in older usage, in Latin spectrum means image or apparition, including the meaning spectre. Spectral evidence is testimony about what was done by spectres of persons not present physically and it was used to convict a number of persons of witchcraft at Salem, Massachusetts in the late 17th century. The word spectrum was used to designate a ghostly optical afterimage by Goethe in his Theory of Colors and Schopenhauer in On Vision. The prefix spectro- is used to form words relating to spectra, for example, a spectrometer is a device used to record spectra and spectroscopy is the use of a spectrometer for chemical analysis. In the 17th century the word spectrum was introduced into optics by Isaac Newton, soon the term referred to a plot of light intensity or power as a function of frequency or wavelength, also known as a spectral density plot. The term spectrum was expanded to apply to other waves, such as sound waves that could also be measured as a function of frequency, frequency spectrum and power spectrum of a signal. The term now applies to any signal that can be measured or decomposed along a variable such as energy in electron spectroscopy or mass to charge ratio in mass spectrometry. Spectrum is also used to refer to a representation of the signal as a function of the dependent variable. Devices used to measure an electromagnetic spectrum are called spectrograph or spectrometer, the visible spectrum is the part of the electromagnetic spectrum that can be seen by the human eye. The wavelength of light ranges from 390 to 700 nm

Andromeda is one of the 48 constellations listed by the 2nd-century Greco-Roman astronomer Ptolemy and remains one of …

Johannes Hevelius's depiction of Andromeda, from the 1690 edition of his Uranographia. As was conventional for celestial atlases of the time, the constellation is a mirror image of modern maps as it was drawn from a perspective outside the celestial sphere.

Andromeda as depicted in Urania's Mirror, a set of constellation cards published in London c. 1825, showing the constellation from the inside of the celestial sphere

Photo of the constellation Andromeda, as it appears to the naked eye. Lines have been added for clarity.

A Chandra X-ray Observatory image of the Sirius star system, where the spike-like pattern is due to the support structure for the transmission grating. The bright source is Sirius B. Credit: NASA/SAO/CXC.

An artist's impression of Sirius A and Sirius B. Sirius A is the larger of the two stars.

Comparison of angular diameter of the Sun, Moon and planets. To get a true representation of the sizes, view the image at a distance of 103 times the width of the "Moon: max." circle. For example, if this circle is 10 cm wide on your monitor, view it from 10.3 m away.

Frequency is the number of occurrences of a repeating event per unit of time. It is also referred to as temporal …

Modern frequency counter

As time elapses—here moving left to right on the horizontal axis—the five sinusoidal waves vary, or cycle, regularly at different rates. The red wave (top) has the lowest frequency (i.e., cycles at the slowest rate) while the purple wave (bottom) has the highest frequency (cycles at the fastest rate).

The Book of Fixed Stars (Arabic: كتاب صور الكواكب‎ kitab suwar al-kawakib) is an astronomical text written by Abd …

''The Great Bear''. The familiar seven stars of the "Big Dipper", recorded by Ptolemy, are visible in the rump and tail, but notice they occur as a mirror-image of what we actually see because Al Sufi provided two images of each constellation, one as we see it in the night sky and one as seen here on a celestial globe. The image is from the copy in the Bodleian Library, the oldest copy extant.

In physics, redshift happens when light or other electromagnetic radiation from an object is increased in wavelength, …

Rendering of the 2dFGRS data

Spectral lines in the visible spectrum of a supercluster of distant galaxies (right), as compared to absorption lines in the visible spectrum of the Sun (left). Arrows indicate redshift. Wavelength increases up towards the red and beyond (frequency decreases).